Movements induced by optic flow in relation to HINE

Abstract

Early recognition of developmental disorders is key to initiating effective physiotherapeutic intervention. The literature emphasizes the importance of visual perception and eye-hand coordination in motor development. This study aimed to determine whether the number of limb movements evoked by visual flow at 3 months of age correlates with HINE scale scores and can predict motor development by month 4. Twenty-nine infants (12 girls, 17 boys) born at term without congenital anomalies or neurological disorders were included. In the third month, motor responses to static and moving images (a sliding checkerboard) were recorded, focusing on the number of limb movements, movement cycles and head movements. At the same time, a HINE assessment was carried out and repeated in the fourth month. A significantly higher number of movements at the moving stimulus was found (p < 0.05). The number of hand movements correlated positively with muscle tone and total HINE score (rho ≈ 0.4). Most infants improved HINE scores by month 4, especially in posture and reflexes, but children with less improvement were noted to have fewer limb movements by month 3. The optical flow method may be a promising tool to aid in the early diagnosis of eye-hand coordination and capture infants at risk for poor gross motor development.

Introduction

Research into the perception of movement in humans dates back to the 1960s¹. This knowledge has helped to improve mechanical processes to mimic human perceptual abilities^2,3. Researchers in psychology and neuroscience have shown particular interest in this subject. Previous reports emphasize that infant motor development can be impaired by numerous factors, such as prematurity, congenital cardiovascular defects or unfavourable environmental conditions^4–6. For example, Sprong and colleagues⁴ demonstrated delays in motor development in children after cardiac surgery, while Zuccarini et al.⁵ demonstrated that early motor competence can affect later cognitive functioning. Additionally, Kumwenda et al.⁶ observed a significant effect of nutrition, including breastfeeding, on the dynamics of motor development in the first months of life. Beyond biological determinants, enriched visual environments, availability of mobile toys, and stimulating home conditions have also been reported to enhance motor coordination, cognitive development, and neurological maturation^7,8. Thus, when assessing motor development in early infancy, it is essential to consider both physiological parameters and the quality of environmental stimulation provided to the child.

In light of these studies, there is an emerging need to develop and validate tools to reliably analyze infants’ motor activity as early as 3 months. The optical flow method, which is used, among others, in video image analysis and robotics, can be used to identify specific movement patterns of infants’ limbs and the trunk, which can provide predictive information about the further course of motor development⁹.

The present study aims to compare the assessment results of the number of upper and lower limb movements induced by optic flow with the child’s motor development study assessment using the Hammersmith Infants Neurological Examination (HINE)^10,11 scale in infants at 3 months of age. Such a comparison allows a preliminary assessment of the usefulness of optic flow as a complementary tool in diagnosing early motor disorders. In addition, it was determined whether the higher number of movements induced by optic flow is associated with the child’s faster/more development of gross motor skills at 4 months of age.

In this study, we utilized the Hammersmith Infant Neurological Examination (HINE), a validated clinical tool widely used to assess early neurological development. While frequently recognized for motor assessment, HINE comprehensively evaluates multiple aspects of neurological function, including cranial nerve integrity, posture control, muscle tone, primitive reflexes, and quantitative and qualitative motor activity. Therefore, it provides valuable insights into the overall neurological status of infants¹⁰.

Sense of vision in infancy

The correct development of the child requires the harmonious cooperation of all the senses, including vision, which is crucial for forming movement perception. Visual information signals the speed and direction of moving objects, which influences the development of motor coordination and motor control. The senses are already developing in the prenatal period, but vision is least stimulated before birth. Only after birth does the child begin to receive visual stimuli intensively, and the child’s ability to see improves during the first year of life¹². During this time, the infant’s vision gradually becomes similar to an adult’s, although refining visual function continues^13,14.

Newborns react most intensely to light and moving objects, with the initial perception of images being unclear. In the first two to three months of life, the child focuses his or her gaze primarily on the contours of objects in the peripheral visual field^15–17.

Then, increasingly better visual perception is formed with the ability to focus on central objects. Research indicates that during this period, infants prefer faces, especially the face of the primary caregiver, which promotes rapport and early communication^18–20.

A key contribution to the perception of visual stimuli is made by saccadic eye movements - rapid, involuntary movements that allow exploration of key elements of the environment^15,16. The first saccadic movements occur in the first weeks of a baby’s life^21,22.

In primate studies, the centres responsible for perceiving and processing information about speed and direction of movement are the primary visual fields (V1) and area MT in the medial temporal lobe. Their role has subsequently been confirmed in studies. Damage to these regions leads to abnormalities in movement perception^22,23.

Understanding how an infant’s visual abilities develop is particularly important in the context of optic flow-based motor analyses. This investigation allows a more precise assessment of whether a child’s movements at 3 months adequately respond to visual stimuli and how this translates into later gross motor skills. That is why it is helpful to consider the stages of visual maturation when interpreting the results of studies in which visual stimuli are a key trigger for motor activity of varying intensity to these stimuli.

Motor development of the child up to 4 months and physiotherapeutic assessment

In the physiotherapeutic assessment of infants, both quantitative and qualitative analysis of motor skills is crucial. Physiotherapists verify whether the child is reaching age-appropriate milestones in large and small motor skills and whether the movement patterns are qualitatively correct. Special attention is paid to so-called “milestones”, at which specific motor skills are expected to emerge²⁴.

As shown by Prins and colleagues²⁵, infants born prematurely are more likely to experience delays in achieving milestones. Therefore, early detection of possible deficits and implementation of physiotherapeutic intervention is important and can significantly improve further development^26,27.

During the first weeks of life, the infant’s motor reactions are mainly based on reflexes. Gradually, however, these innate patterns give way to purposeful movements²⁸. Around 3 months of age, initial stabilization of the head and neck begins to become apparent^27,29. In contrast, by the end of the third month, most babies can lie on their stomachs, supporting themselves on their forearms with their elbows in front of the shoulder line³⁰. In the supine position, the child can bring the knees close to the abdomen and lift the arms above the chest, indicating the successive formation of axial body alignment and the gradual counteraction of gravity.

In order to objectively assess these skills and catch possible abnormalities, standardized tools such as the Hammersmith Infant Neurological Examination (HINE) are used. This scale includes an assessment of muscle tone, cranial nerve function, reflexes and posture, awarding points from 0 to 3 for each category^10,11,31,34. A score below the established norm may suggest the presence of a neurological disorder and indicate the need for further observation or intervention^32,33.

A child’s normal physical and motor development depends heavily on visual stimuli. Early eye-hand coordination and the ability to track moving objects can promote the faster emergence of goal-directed movements. Therefore, when analyzing an infant’s motor activity using visual flow, it is important to consider the degree of development of fine motor skills and visual perception during the key months of life.

Methods and research group

The study had a two-stage design. Stage I was conducted with infants aged 3 months (8.−12 weeks). Motor responses to two visual stimuli types were recorded: static (background with no moving element) and moving, evoking visual flow.

From the recordings, the amount of movement of the upper limbs, lower limbs and head in response to the visual stimulus was analyzed. During this stage, development was also assessed using the standardized Hammersmith Infant Neurological Examination (HINE) questionnaire.

Stage II, conducted when the infants were 4 months old, involved only a reassessment of gross motor skills using the HINE questionnaire¹¹ - this time without simultaneous analysis of the optic flow response. This approach made it possible to compare the intensity of movements (recorded at 3 months of age) with the results of the gross motor assessment collected at both time points. It also determined to what extent the lower increase in HINE scores between 3 and 4 months of age could be related to the low number of limb and head movements observed at 3 months. A schematic of the study methodology is shown in Fig. 1.

Fig. 1.

Study scheme and number of children covered.

The present study continues previous analyses and includes 29 infants (12 girls, 17 boys). A previous publication entitled Analysis of Upper and Lower Limb Movement in Infants in Response to Optic Flow - Development of a Methodology and Results of a Preliminary Study showed that a moving image triggers a stronger motor response than a still image. Stronger motor response was also observed in older infants (9.−10 weeks) compared to younger infants (6–7 weeks).

The present study aimed to determine whether there is a correlation between the number of limb movements (upper and lower) and head movements and the results of gross motor assessment (using HINE) in infants at 3 months of age. In addition, it was investigated whether tests using optic flow could serve as a tool for the early assessment of eye-hand coordination and to help predict an infant’s weaker development by 4 months of age. To this end, two research questions were posed:

1. Is there a correlation between the number of upper and lower limb movements and gross motor assessment scores in infants at 3 months?

2. May optic flow tests provide an early tool for assessing eye-hand coordination in infants scoring lowest on the HINE questionnaire by 4 months of age?

Study methodology

Qualification for the study

The study was conducted in a specialized rehabilitation clinic using the workstation described in a previous publication³⁴. The initial selection of children (n = 39) was based on a medical interview with parents/guardians conducted during the first stage of the study (8–12 weeks of age). The inclusion criteria for the study comprised full-term birth (defined as ≥ 38 weeks of gestation), a normal APGAR score (≥ 8) at both 1 and 5 min after delivery, and a birth weight appropriate for gestational age. Only infants without congenital abnormalities or perinatal complications were considered eligible. Additional requirements included symmetrical head posture during the initial clinical examination, as well as the ability to fixate and follow a visual stimulus. Participation in the study was also conditional on obtaining written informed consent from the parents or legal guardians. The exclusion criteria encompassed preterm birth (< 38 weeks of gestation), low APGAR scores (< 8), and the presence or suspicion of congenital malformations or structural anomalies. Infants with diagnosed or suspected cardiac, genetic, metabolic, or neurological disorders were excluded, as well as those with developmental delays or visual disturbances. Furthermore, children exhibiting abnormal positioning responses, such as torticollis, as assessed during physiotherapeutic evaluation, were not eligible for the study. Lack of parental consent also constituted grounds for exclusion.

Finally, 39 infants (mean age: 10 ± 0.5 weeks) were enrolled in the first stage of the study. Each parent/guardian gave written informed consent for their child’s participation in the study.

The study was approved by the Bioethics Committee of the Poznan University of Medical Sciences (approval no. 701/20, dated November 4, 2020) and conducted in accordance with the Declaration of Helsinki. Written informed consent was obtained from the parents or legal guardians of all participants.

The sample size was determined by the number of infants meeting all inclusion and exclusion criteria during the data collection period. No formal sample size calculation was performed, as this was an exploratory study focusing on method development and preliminary associations.

Study procedure

Following the completion of the first stage (surveying children at 8.−10 weeks of age), 29 children who met all the inclusion criteria for the second stage of the study, in which children were over 2 months of age and before reaching 3 months of age (mean age: 10 ± 0.5 weeks), were selected for further analysis. The study procedure included:

Stage I (children aged 3 months):

1. Medical history - Inclusion and exclusion criteria were reviewed again. Parents/guardians confirmed no new cardiac, genetic or neurological diagnoses.

2. Video recordings - These were made using two GoPro 8 cameras (full HD resolution, 60 fps). The total recording time was 20 s. The cameras were placed facing each other to record the left and right sides of the infant’s body.

3. Image presentation - An image of a black and white chessboard (7 × 10 cm) was projected onto a semi-transparent tabletop underneath, which provided high contrast and prevented blinding the child. Two projections were studied:

• E1 (static): stationary chessboard;

• E2 (dynamic): The chessboard moved at a velocity of 0.17 m/s towards the child. This specific speed was selected based on preliminary pilot testing³³ and existing literature⁹ suggesting that moderate movement speeds are optimal for capturing infants’ visual attention and triggering motor responses without inducing distraction or overstimulation. The interval between E1 and E2 was approximately 20 s.

The parents/guardians were in the same room but outside the child’s field of vision.

• 4.Supporting infants for visual stimulus presentation.
• 5.During the visual stimulus presentation, the therapist held the infant supine (Fig. 2 illustrates the grip used during this phase with a mannequin model). One hand stabilized the thorax without restricting the movement of the upper limbs, and the other supported the pelvic region, allowing observation of spontaneous motor responses of the head, trunk and limbs.

Fig. 2.

Demonstration of the standard grip for head stabilization using a doll model: (a) top view, (b) side view.

• 6.VIDEO analysis − 20-second excerpts from the video were assessed by counting the number of flexions and extensions of the upper and lower limbs. In addition, it was noted whether flexion/extension of the fingers and toes occurred. An alternating sequence of upper and lower limb movements resembling crawling was defined as a ‘cycle’.
• 7.Infant motor development assessment - This was carried out using the Hammersmith Infant Neurological Examination (HINE) questionnaire in supine and prone positions. The same physiotherapist conducted the first and second assessments to ensure consistency and minimize interrater variability. All questionnaires were completed by the principal investigator (first author) and were administered in Polish. Results indicating abnormal muscle tone were not grounds for exclusion from the study but were noted on the assessment sheet.

The Hammersmith Infant Neurological Examination (HINE) questionnaire includes 26 components to assess cranial nerves, posture, quantitative and qualitative aspects of movement, muscle tone and reflexes. Each of these 26 tasks is awarded 0 to 3 points, for a maximum of 78 points. It is accepted in the scientific literature that a score of fewer than 57 points may indicate neural coordination disorders and the scale itself has a high diagnostic sensitivity in detecting early neurological abnormalities.

Stage II (children 4 months of age).

• 8.Spontaneous motor skills testing - analogous to stage I was carried out using the Hammersmith Infant Neurological Examination (HINE) questionnaire,
• 9.Statistical analysis - Data were analyzed using Statistica 13. Ordinal variables were presented using median (Me), quartile (Q₁-Q₃) and min-max values. Differences for related variables were tested using the Wilcoxon test. The Mann-Whitney test was used to compare independent results. Correlations between variables were tested with Spearman’s test, and weak correlations were considered with rho values between 0.3 and 0.4. medium correlations with rho between 0.4 and 0.6 and strong correlations with rho > 0.6. The significance of the tests performed was assumed at p < 0.05.

The interval between the first and second examinations was 4 weeks for each child.

Results

Assessment of the amount of limb movement under optical stimulus

The analysis compared the number of upper and lower limb movements, head movements, and coordinated limb cycles in response to static (E1) and moving (E2) visual stimuli. As the same group of children was tested under both conditions, the Wilcoxon test for paired data was used to determine statistically significant differences.

Table 1 presents the descriptive analysis (minimum and maximum values as well as the median) and the results of the Wilcoxon test (with p-values) for comparing the number of limb and head movements between static and moving images. In each category analyzed (arms, legs, movement cycle, head movements), a statistically significant increase in the number of movements was observed in the moving image projection (E2) compared to the static image projection (E1) (p < 0.05).

Table 1.

Descriptive and statistical analysis of the comparison of the number of movements made by the child with the right and left lower and upper limbs, the cycle of limb movement performed and head movements - comparison between the results obtained for the image of the optical stimulus in the still image projection versus the results in the moving image projection.

Body movement	Static image (E1)			Moving image (E2)			Test result	p-value
	Min	Me	Max	Min	Me	Max
Right hand	0	1	4	0	2	5	3.659	< 0.001
Left hand	0	1	4	0	3	5	3.696	< 0.001
Right leg	1	2	5	0	3	7	2.726	0.006
Left leg	1	2	5	0	3	7	3.051	0.002
Cycle (simultaneous movement of arm and leg)	0	1	2	0	1	3	3.180	0.001
Head movements	0	1	2	0	1	5	1.960	0,050

Min (Minimum); Me (Median); Max (Maximum).

Wilcoxon’s test.

Figure 3 shows the median number of movements with rank interval (Q1-Q3) and the minimum and maximum range for projections E1 and E2. The highest percentage increase was recorded in upper limb movements, where the increase in the mean reached more than 50%.

Fig. 3.

Results of the number of movements during projection with still and moving image.

The results indicate that the optical stimulus in the moving projection significantly increased the movements of the upper and lower limbs on the right and left sides of the body. In addition, an increase in the number of simultaneous movements (cycles) of the lower and upper limbs was shown. An increase in the number of movements of the child’s head during the projected image was also shown.

Results of the HINE questionnaire

Table 2 shows children’s scores obtained in the HINE questionnaire at 3 and 4 months of age. No significant changes were observed in the Cranial Nerves subscale, as the minimum, median and maximum values remained stable (13, 15 and 15 points, respectively). Similar stability was observed in the Quantity-Quality area, where the range was 4–6 points in both measurements, and the median remained equal to 5.

Table 2.

Descriptive and statistical analysis of the comparison of the number of movements made by the child with the right and left lower and upper limbs, the cycle of limb movement performed and head movements - comparison between the results obtained for the image of the optical stimulus in the still image projection versus the results in the moving image projection.

HINE components	3 month old children			4 month old children			Test result	p-value
	Min	Me	Max	Min	Me	Max
Cranial nerves (max = 15)	13	15	15	13	15	15
Body posture (max = 18)	10	15	16	10	16	18	4.457	< 0.001
Quantity-Quality (max = 6)	4	5	6	4	5	6
Tension (max = 24)	11	21	23	11	21	24	3.195	0.001
Reflexes (max = 15)	5	8	9	6	10	12	4.372	< 0.001
HINE TOTAL	46	63	68	47	67	74	4.457	< 0.001

Min (Minimum); Me (Median); Max (Maximum).

Wilcoxon’s test.

In contrast, apparent differences emerged in the Body Posture subscale, where the median increased from 15 to 16 points (p < 0.001). In the Tension area, statistical significance (p = 0.001) was observed despite an unchanged median (21 points), suggesting a change primarily in the distribution of extreme scores. In the Reflexes subscale, the median increased from 8 to 10 points (p < 0.001).

Analysis of the overall HINE scores also indicated a significant difference between 3 and 4 months of age (p < 0.001). This change translated into an increase in the median score from 63 to 67. It should be noted, however, that although improvements were observed in most infants, detailed analysis revealed variability in the magnitude of these improvements. Specifically, out of the 29 infants examined, three children’s scores (10.3%) remained unchanged, one child (3.4%) showed a slight increase of only 1 point, two children (6.9%) improved by 2 points, and the remaining 23 children (79.3%) demonstrated an improvement of 3 or more points (Fig. 4).

Fig. 4.

HINE questionnaire results from children aged 3 and 4 months.

The observed differences between 3 and 4 months of age in most HINE subscales indicate a progressive improvement in neurological parameters, especially in the subscales of posture and reflexes. The stability of the scores concerning cranial nerves and quantitative and qualitative aspects of movement suggests a relatively even development of these functions. The significant increase in the total score, however, confirms the usefulness of the HINE scale in monitoring dynamic changes during the first months of a child’s life.

However, a group of infants did not show marked improvement, indicating the need for further observation and possibly more in-depth diagnostics. A below-normal score (i.e. <57 points) was found in three children at 4 months of age, while this was the case for four infants at 3 months. All parents were informed of the need for neurological consultation. Ultimately, two children were diagnosed with reduced muscle tone in the body axis (axial hypotonia) but without features of cerebral palsy; in one case, no further information on further diagnostic management was obtained. These findings highlight the importance of regular neurological follow-up in infants with borderline or reduced HINE scores.

Correlation analysis of results from stage I

Spearman’s rank correlation analysis was performed to determine the relationship between children’s scores for the HINE questionnaire at 3 months of age and the number of limb movements, movement cycles and head movements. The test was performed separately for the following conditions:

• E1 (still image) - results included in Table 3;

• E2 (moving image) - results included in Table 4.

Table 3.

Cross-correlation analysis for subscales from the HINE questionnaire (cranial nerves, posture, quantity-quality, tension, reflexes) and final values against scores for the number of children’s limb movements, cycles of movement and head movements when projected with a still image.

Variables	Right hand	Left hand	Right leg	Left leg	Cycle	Head movements
Cranial nerves (max = 15)	0.15	0.19	0.11	0.04	0.21	0.33*
Body posture (max = 18)	−0.07	−0.01	−0.36*	−0.38*	−0.15	0.02
Quantity-Quality (max = 6)	0.12	0.16	−0.15	−0.24	−0.15	−0.12
Tension (max = 24)	0.11	0.17	−0.10	−0.10	0.02	0.04
Reflexes (max = 15)	−0.29	−0.32*	−0.23	−0.25	0.05	−0.06
HINE TOTAL	0.05	0.09	−0.24	−0.27	−0.07	−0.01

Spearman’s rang correlation test; p < 0,05 - *.

Table 4.

Cross-correlation analysis for subscales from the HINE questionnaire (cranial nerves, posture, quantity-quality, tension, reflexes) and final values against the results of the number of movements of the child’s limbs, cycles of movement, and head movements on moving image projection.

Variables	Right hand	Left hand	Right leg	Left leg	Cycle	Head movements
Cranial nerves (max = 15)	0.25	0.16	−0.10	−0.10	0.16	0.02
Body posture (max = 18)	0.16	0.16	−0.06	−0.06	0.12	−0.20
Quantity-Quality (max = 6)	0.26	0.29	0.04	0.13	0.16	−0.12
Tension (max = 24)	0.46 *	0.38 *	0.14	0.08	0.08	−0.12
Reflexes (max = 15)	0.09	0.25	0.11	0.14	0.20	−0.16
HINE TOTAL	0.37 *	0.35 *	0.05	0.03	0.09	−0.20

Spearman’s rank correlation; p < 0,05 - *.

In condition E1 (Table 3), Spearman’s rank correlation analysis revealed mainly weak negative correlations between some subscales of the HINE questionnaire and the number of lower limb movements. This observation was particularly evident for the Posture subscale, which showed correlations of r=−0.36 (right leg) and r=−0.38 (left leg). In contrast, the Cranial Nerves subscale positively correlated with head movements (r = 0.33), suggesting a potential link between cranial nerve control and head activity.

In contrast, the number of left-hand movements correlated marginally with the Reflexes subscale (r=−0.32). However, no significant correlation was observed between the number of movements (both upper limbs, lower limbs and head) and the total HINE score, which may indicate the relative independence of these parameters in the conditions studied. Other subscales, such as Quantity-Quality or Tension, also showed small and statistically insignificant correlations with limb or head movement.

The lack of significant correlation with the HINE total score means that the recorded limb and head movements do not directly reflect the global neurological assessment, which may require further in-depth studies in a larger study group.

In condition E2 (Table 4), the correlations between the number of movements and the HINE subscales were more potent than in condition E1 (Table 3). The highest correlation coefficients were recorded between the number of upper limb movements and muscle tension and the HINE total score. For the right hand, these values were r = 0.46 and r = 0.37 (p < 0.05), respectively, while for the left hand, they were r = 0.38 and r = 0.35 (p < 0.05). Thus, increased upper limb movement intensity reflects higher muscle tone and the child’s overall neurological score. Elsewhere, the correlations were weaker or insignificant, indicating no clear relationship between the number of movements (right leg, left leg, movement cycle, head) and the HINE subscales.

Compared to E1, where correlations tended to be weak and often negative, a positive relationship, particularly between hand movements and muscle tone and overall HINE score, is evident in the E2 condition. These results suggest that the moving visual stimulus may engage the infants’ neuromotor system more strongly, as reflected in the higher correlation coefficients. This observation provides an additional argument for optic flow as a complementary tool in assessing early neurological and motor development parameters.

Assessment of limb mobility against the magnitude of improvement in HINE questionnaire scores

In order to determine whether the magnitude of improvement in the HINE questionnaire between 3 and 4 months of age is related to the frequency of upper limb, lower limb and head movements recorded at 3 months (E2 conditions - moving stimulus projection), the subjects were divided into two groups. The first group consisted of infants with a HINE score gain of 3 or more (n = 23), while the second group included infants who did not show any improvement or had a growth score of 2 or less (n = 6).

Table 5 compares the number of right and left arm, leg, movement cycle (alternating upper and lower limb movement) and head movements between the two groups (Mann-Whitney test). The results showed a statistical trend (p < 0.10) for the number of upper limb movements: right (p = 0.092) and left (p = 0.088), suggesting a possible link between higher hand motor activity at 3 months of age and more significant gains in HINE scores. In contrast, no significant differences were found for leg movements, head movements or the motor cycle (p > 0.10) (Fig. 5).

Table 5.

Descriptive and statistical analysis of the comparison of the number of the right and left upper and lower limbs, the number of cycles and head movements in the third month with the change in HINE scale between the third and fourth month.

Body movement	3 or more HINE improvement points			2 or less HINE improvement points			Test result	p-value
	Min	Me	Max	Min	Me	Max
Right hand	0	2	5	0	1.5	3	1.617	0.092
Left hand	0	3	5	0	1.5	3	1.704	0.088
Right leg	1	3	7	0	3	7	0.165	0.869
Left leg	1	3	7	0	3	7	0.247	0.805
Cycle (simultaneous movement of arm and leg)	0	1	3	0	0.5	2	1.367	0.172
Head movements	0	1	5	1	1	3	−1.099	0.272

Min (Minimum); Me (Median); Max (Maximum).

Mann-Whitney’s test.

Fig. 5.

The number of movements upon optic flow according to alteration in HINE between the third and fourth month.

Summary of the results

The results may indicate that a higher number of upper limb movements in infants under the influence of an optical stimulus at 3 months of age is partially associated with a higher rate of improvement in the HINE questionnaire. On the other hand, the lack of a similar relationship in the number of lower limb and head movements underscores the need for further research in a larger study group to clarify the specificity of early eye-hand coordination in the context of gross motor development. Since this was an exploratory study, no formal power analysis was performed. The non-parametric methods used are appropriate for small samples, but future studies with larger cohorts are needed to validate these findings.

Perception of motion plays a key role in daily human functioning, enabling the evaluation of the speed and direction of moving objects. The scientific literature on motion perception often uses optic flow analysis to study infants’ responses. The results of our study confirm that infants respond more intensely to a moving visual stimulus than to a static stimulus. This phenomenon is consistent with previous reports indicating early development of eye-hand coordination².

Our findings regarding increased limb movements in response to optic flow stimuli align with existing evidence highlighting the critical role of visual and physical stimulation in the early months of life. Prior research underscores that visually enriched environments and the presence of mobile toys positively correlate with improved motor coordination and cognitive outcomes in infants²⁶.

Along similar lines are the observations of Marianne Barbu-Roth, who noted that even children younger than 8 weeks show a markedly higher number of movements in response to a moving image compared to a stationary stimulus³⁶. In addition, numerous other studies confirm that infants^15,16,27 respond more intensely to dynamic stimuli from the earliest weeks of life, providing a basis for further research into motion perception and its relationship to motor development.

In the present study, it became equally clear that a moving projection of a black and white checkerboard (so-called optical flow) provokes significantly higher levels of motor activity than a static projection. From the point of view of early diagnosis of motor disorders, such an observation is particularly valuable, as it indicates that differences in reactivity to visual stimuli can be observed even in the first months of life. These findings provide a rationale for optic flow as an additional tool for assessing eye-motor coordination.

While this study offers preliminary insights into the early sensorimotor responses elicited by optic flow and their short-term associations with HINE scores, the one-month follow-up period limits conclusions regarding these responses’ persistence or developmental consequences. A longer-term longitudinal approach would allow researchers to assess whether early motor reactions to visual motion stimuli predict later gross motor milestones, neurological outcomes, or neurodevelopmental disorders. The extended observation would also help evaluate the clinical utility of optic flow measurements in early screening frameworks.

Several standardized scales, such as the Hammersmith Infant Neurological Examination (HINE), the Alberta Infant Motor Scale (AIMS) or others, are used to assess infant motor development. In the present study, we primarily used the HINE scale, which is considered a reliable tool for the early assessment of neurological development. For example, Uusitalo et al. found that higher HINE scores in children born prematurely at age 2 correlated with higher intelligence levels at age 11⁴. In contrast, other studies have confirmed the effectiveness of combining HINE, Prechtel’s assessment of global movement patterns and neuroimaging (MRI/USG) results in the reliable and early diagnosis of cerebral palsy; the combined sensitivity and specificity for detecting cerebral palsy was as high as 98–99%⁵. Many other studies have compared the results from the HINE test with other parameters^37,38. In the present study, we observed the average power of correlating the results of the HINE sum and the component of muscle tone and reflexes with the amount of upper and lower limb movement during a moving visual stimulus presented.

Interestingly, while the moving stimulus condition (E2) revealed statistically significant correlations between limb movement and both the HINE total score and subcomponents (especially muscle tone), no such associations were observed under static conditions (E1). Spontaneous responses to static stimuli may not fully reflect neurological maturity measured by clinical scales. In contrast, the dynamic visual input in E2 engages sensorimotor systems more robustly, eliciting responses that align more closely with the neurological domains captured by the HINE. These results suggest that optic flow may selectively activate pathways involved in purposeful or integrated motor responses, strengthening its case as a specific and sensitive complementary tool rather than a passive observation of movement frequency.

While most infants in our study demonstrated progressive improvement in neurological function, as measured by HINE between 3 and 4 months of age, a subgroup of six children showed minimal to no change (an increase of only 0–2 points), and three children had scores below the accepted threshold of 57 points. Notably, two of these infants were subsequently diagnosed with axial hypotonia. These findings underscore the importance of interpreting HINE scores in terms of normative thresholds and the context of the rate of change over time. Early neurological development can be non-linear and heterogeneous, even among healthy infants.

In this context, optic flow responsiveness may offer added value. The ability to detect increased limb activity in response to moving visual stimuli may help reveal early differences in sensorimotor integration before they become apparent in standardized assessments. Thus, optic flow analysis could serve as a complementary tool to identify subtle deviations in developmental trajectories and support earlier referrals for follow-up diagnostics or intervention.

Several studies on early motor reflexes draw attention to a child’s weight as a factor affecting the ability to maintain or initiate specific motor sequences, such as the stepping reflex³⁹. Indeed, heavier infants may have difficulty supporting their weight, sometimes misinterpreted as a reflex loss. Also, in the present study, body weight (birth and current) was analyzed to investigate the correlation of this variable with the number of movements during the projection of a visual stimulus⁵. Although there was no apparent effect of body weight on motor activity in the study group, this needs further verification on a larger sample and over a more extended period of observation. Although no significant correlation was found between motor activity and either birth or current body weight in this sample, this result should be interpreted cautiously. Previous studies have indicated that body mass may influence early reflexive or postural motor behaviours, such as the stepping reflex or head control, by modulating biomechanical load and muscle recruitment (e.g., heavier infants showing reduced stepping frequency). The lack of such an effect in our data may reflect sample limitations rather than a genuine absence of association. Further research involving larger and more heterogeneous groups is needed to determine whether body weight interacts with visual-motor integration in infancy and to identify any potential thresholds or modulating factors. Furthermore, the present study focused on quantifying gross motor activity but did not attempt to categorize specific movement patterns systematically. Previous research, such as Gima et al.⁴⁰, has emphasized the diagnostic potential of subtle, spontaneous movement features, including postural asymmetries and head position dynamics, in identifying early motor signs of neurodevelopmental disorders like ASD. Similarly, Kihara et al.⁴¹ developed the Infant’s Behaviour Checklist, linking specific motor and behavioural features in low birth weight infants to later neurodevelopmental outcomes. Incorporating such frameworks in future studies could enhance the interpretation of spontaneous movement patterns observed in response to optic flow stimuli. It may offer more nuanced early markers of neurological function.

In conclusion, the results demonstrate the significant influence of a dynamic visual stimulus (optic flow) on infant motor responses, suggesting its potential role as a complementary method in early neurological assessment. Most infants showed stronger motor responses to moving images, indicating enhanced sensorimotor engagement compared to static conditions. These findings support incorporating visual motion stimuli into early diagnostic frameworks and highlight the importance of individualized sensory environments. Future research should explore how optic flow-induced behaviours relate to long-term developmental trajectories, with special attention to home-based and environmental factors.

Conclusions

1. Influence of optic flow - Infants aged 3 months show more intense motor activity in response to a moving visual stimulus than to a static stimulus, confirming the usefulness of optic flow in studying eye-motor coordination at an early stage of development.

2. Correlations with selected HINE parameters were observed mainly with a moving stimulus, especially concerning muscle tone and summation scores.

Additionally, although birth and current body weights were considered in the analysis, no significant correlation was found between these variables and the number of limb movements evoked by the visual stimulus. These findings suggest that weight did not substantially influence motor responsiveness within our sample’s observed range of body weight. However, given previous reports linking physical growth to motor patterns in early infancy, body weight remains a potentially important variable. It should be controlled and further explored in future studies with larger cohorts.

The results support the need for further in-depth research involving a larger and more diverse infant population and extended longitudinal follow-up. Such an approach would provide a more comprehensive understanding of optic flow responses’ role in early motor development and their potential to complement traditional diagnostic tools. Future studies should determine whether early reactivity to dynamic visual stimuli can predict long-term motor or cognitive outcomes, thereby enhancing early detection and intervention strategies.

Limitations of the study

The group was not significant and limited to healthy children who all showed similar reactions; no abnormalities were observed. Future research should include infants with a broader range of neurological and motor profiles to evaluate the potential of optic flow as a complementary diagnostic tool.

The forced position during the test could have caused the motor response in addition to the visual stimulus.

The relatively small sample size may limit the generalizability of the findings and the statistical power of subgroup analyses. Moreover, since this was an exploratory study, no formal power calculation was performed. Although appropriate non-parametric methods were applied, the limited sample size may have reduced the ability to detect weaker or more nuanced effects. Future studies with larger cohorts are therefore needed to validate and expand upon these results.

Furthermore, this study did not control or record some potentially influential factors—such as parental education level and the broader home environment. While body weight was included in the analysis, its effects were insignificant in this sample; however, it remains a variable of interest for future studies.

Author contributions

Żaneta Pawlak-Andryszczyk designed the study, collected the literature, conducted the research, and wrote the initial manuscript draft. Marek Andryszczyk performed the analyses, gathered additional sources, wrote part of the text, and prepared the final version of the manuscript. Magdalena Sobieska edited the text, verified its scholarly accuracy, and made the final revisions. All authors reviewed and approved the final version of the manuscript.

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Data availability

The datasets generated and analyzed during the current study are not publicly available due to privacy and proprietary restrictions. However, they can be provided by the corresponding author upon reasonable request: Video recordings of infant motor responses, supporting Figs. 2, 3 and 5; Tables 1, 3, 4 and 5, are available upon request from Marek Andryszczyk at marek.andryszczyk@ukw.edu.pl or Żaneta Pawlak-Andryszczyk ze.pawlak@gmail.com. HINE assessment scores (anonymized) and raw movement data used in statistical analysis can also be shared upon request, subject to compliance with institutional ethical guidelines (supporting Figures 4; Table 2).Researchers interested in accessing these datasets are encouraged to contact the corresponding author via email at marek.andryszczyk@ukw.edu.pl or ze.pawlak@gmail.com The data will be made available in a suitable repository upon request.

Declarations

Competing interests

The authors declare no competing interests.