Fig. 2.
Perceptual tasks (a–c), psychophysical responses (d–f) and cellular responses (g–i) analysed in the text. (a–c) Stimulus schemata for the three tasks considered (adapted from Rees et al. (2000), Romo et al. (2002), Micheyl et al. (2005)). (a) Three different levels of coherence in the visual motion task. At 0% coherence, all dots move randomly. As coherence increases, more dots move in a coherent fashion in one direction (black symbols) while the others move randomly. (b) Somatosensory task, showing a frequency discrimination trial where the second stimulus (f2) has a higher frequency than the first stimulus (f1). (c) Auditory percepts during stream segregation. The stimuli (repeating triplets of different frequency tones, ABA) are perceived as a single stream of connected tones (solid lines) or as two monotonic streams with different tempi playing in parallel (dashed lines). (d–f) Psychometric curves for visual motion (d), somatosensory (e), and auditory (f) tasks (from Britten et al. (1992), Romo et al. (2002) & data in Micheyl et al. (2005) provided by C. Micheyl respectively; curves in d and e are fits of Eq. (1) of Materials and methods). Black lines indicate threshold values for perception, for the random dot coherence, difference in flutter frequency (f2–f1), and time for build-up of auditory stream segregation (but in d and e the exemplar thresholds differ from the mean values used in the calculations). (g–i) Neuronal responses evoked by the tasks (adapted from Britten and Newsome (1998), Romo et al. (2002), Micheyl et al. (2005)). (g) Direction tuning functions for an MT neuron at motion coherences of 12.8, 25.6, 51.2, and 100% (bottom to top; frequency scale adjusted so that background firing rate (8 Hz) and rate with a 0% coherence stimulus (20 Hz) are those found on average: see text). For stimuli close to the preferred direction, increased coherence increases firing, but as the orientation approaches 180o to the preferred direction, the rate decreases below the rate with a 0% coherence signal (arrows). (h) Average firing rate parameter a2 (see Materials and methods) for S2 neurons, derived by fitting the Eq. firing rate = a1⁎f1 + a2⁎f2 + constant, during f2 presentation. The dependence of the firing rate on f2 changes over time. ( i ) Spike counts (per 125 ms) evoked in A1 neurons with best frequency A, by the A and B tones in repeating ABA triplets.