Published in Annual Meeting, Neuroscience, Oct 2015.
The visual world is richly decorated with texture, which often serves to delineate important elements of natural scenes. In anesthetized macaque monkeys, selectivity for the key statistical features of natural texture is weak in V1, but substantial in V2, suggesting that neuronal activity in the second visual area might directly support texture perception. To test this hypothesis, we investigated the relation between single cell activity in macaque V1 and V2 and simultaneously measured psychophysical judgments of texture. We generated stimuli along a continuum between naturalistic texture (synthesized using a well-known model - Portilla & Simoncelli, 2000) and phase-randomized noise. We trained two macaque monkeys to judge whether a sample texture (presented for 500 ms in the near periphery) more closely resembled a fully naturalistic or phase-randomized texture. To enable a direct comparison of neuronal and behavioral sensitivity, we asked the monkey to discriminate the texture type (chosen from a set of five) for which a concurrently recorded neuron showed maximal selectivity. Both animals performed the task well, with discrimination thresholds rivaling those of human observers. Under these conditions, some V1 neurons (20/72) and many V2 neurons (44/87) were tuned for naturalness. Selectivity for naturalness evolved with a time course that differed across areas. In V2, selectivity emerged early and peaked 50 ms after the initial transient. In V1, selectivity was initially absent, but increased gradually over 400 ms. Ideal observer analysis of neuronal responses revealed that single V1 and V2 neurons carried much less information about texture naturalness than the animals' behavioral reports. On average, psychophysical sensitivity exceeded neuronal sensitivity by a factor of 12 in V1 and a factor of 8 in V2. Moreover, fluctuations in the activity of single V2 neurons across repeated presentations of the same stimulus did not predict fluctuations in behavioral choice. Together these results suggest that texture perception arises from the combined activity of many V2 neurons.