Table 1.

Cognitive/behavioral phenotypes and putative neural correlates of fragile X syndrome and Williams syndrome

	Cognitive/behavioral features			Putative neural correlate
	Fragile X syndrome		Williams syndrome	Fragile X syndrome		Williams syndrome
Social		Similarities:			Similarities:
		Deficits in recognizing subtle social cues and impaired effectiveness in social interaction with peers.			Amygdala and fusiform cortex are affected, albeit in different directions.
	Contrasts:			Contrasts:
	Avoidance of social interactions [13]. Poor eye contact [15]; gaze aversion [16]. hcreased social anxiety [15].		Hypersociability [17,18]; approachable [19]; greater attention bias towards happy faces than angry faces [21]. Difficulty in disengaging eye contact in children and adolescents [20]. Ieased phobic like-fears and general anxiety [23].	Smaller amygdala [24▪,25]. Greater sensitization in the left amygdala with successive exposure to direct gaze [16]. Reduced response to facial expressions within the fusiform cortex [32].		Larger amygdala [26,27]. Reduced amygdala response to fearful facial expressions [28,29]. Abnormally greater response to facial expressions within fusiform cortex face area [31▪]. Right lateral OFC activation in response to positive faces [33]; right medial OFC activation in response to negative faces [33].
Response inhibition		Similarities:			Similarities:
		Both FraX [34,35] and WS [36] show deficits in response inhibition.			Dysfunction of frontostriatal circuitry (FXS [38]; WS[39]), albeit in opposite directions.
	Contrasts:			Contrasts:
	Response inhibition is dysfunctional in male [35] and female [34] adolescents. Deficits in disengaging attention and set-shifting in toddlers [37].		Difficulty inhibiting inappropriately high appetitive drive towards social interaction [36]. Deficits in selective attention in toddlers [37].	Reduced volume of frontal lobe [30▪]. Dramatically increased caudate nucleus volume [25,40,41,42▪▪,43]. Ineased density of white matter tracts in left ventral frontostriatal pathway [38]. Significant correlation between FMRP and neural responses in right VLPFC and both left and right striatum [34].		Disproportionately large frontal areas [44]. Reduced volume of caudate nucleus [45].
Visuospatial		Similarities:			Similarities:
		Impairments in visuospatial working memory tasks (FraX [8]; WS [46]) and visuomotor coordination (FraX [2]; WS [47,48]).			Deficits in the magnocellular/dorsal pathway functioning (FraX [49,50]; WS [51,52]). Decreased activation in SPL during visuospatial working memory task (FraX [53]; WS [54]).
	Contrasts:			Contrasts:
	Deficits in lower level visual processing have been rarely documented, except for aberrant saccade behavior [55], which may confound assessment of visuospatial functioning.		Deficits in lower level visualprocessing (strabismus [56], reduced visual acuity [57], reduced stereopsis [58], and impairment in making simple saccades [59]) may confound assessment of visuospatial functioning.	Deficits on tasks with stimuli targeting the magnocellular pathway [49,50], Increased size of IPL [25,30▪]. Reduced IPL activation during visuospatial working memory tasks [53]. White matter connectivity in postcentral gyrus is decreased [60].		DTI reveals that white matter tracts within the magnocellular pathway such as the SLF exhibit an abnormal structural integrity [51,52]. Visuomotor abilities are associated with gray matter density of left IPL [61]. Greater gray matter density within the postcentral gyrus [62].
Language		Similarities:			Similarities:
		Impairments in auditory working memory (FraX [8]; WS [63])			Abnormalities in size of STG and cerebellum, although the deviations from normal are in opposite directions.
	Contrasts:			Contrasts:
	Atypical speech, limited expressive communication.		Unusually loquacious and highly expressive [64].	Small STG volume [32]. Reduced volume of cerebellar vermis [65].		Asymmetric STG [66] and disproportionately larger STG volume [45]. Enlarged cerebellar vermis [67].

DTI, diffusion tensor imaging; FraX, fragile X syndrome; IFC, inferior prefrontal cortex; IPL, inferior parietal lobe; OFC, orbitofrontal cortex; SLF, superior longitudinal fasciculus; SPL, superior parietal lobe; STG, superior temporal gyrus; VLPFC, ventral lateral prefrontal cortex; WS, Williams syndrome.