Neuronal representation of subjective shapes in area V4

Subjective contours, or illusory contours, are a common tool to study neuronal mechanisms of perceptual completion in the visual system. Psychophysical investigations of subjective contours and shapes point toward an early integration of visual cues to form the illusory percept. Furthermore, recordings in monkeys have shown that subjective contours evoke spiking responses from neurons in areas V1 and V2. However, neuroimaging studies in humans suggest that the strongest percept-related responses to subjective contours are found in higher-tier visual areas compared to lower-tier visual areas. A possible explanation for this apparent confound is that early cortical responses to subjective contours are mediated by descending signals from higher level visual areas, such as shape-sensitive area V4. This idea is further supported by the finding that ablation of V4 in macaques results in selective impairment of subjective contour perception.

Here we investigate single neuron responses in macaque area V4 to subjectively defined shapes. We conducted simultaneous multi-electrode recordings using a chronic 96-channel array in two fixating macaque monkeys. Stimuli included conventional subjective squares defined by four “pac-man” inducers with and without coloration, along with physically similar controls that did not induce perception of illusory surfaces. We sampled V4 neurons with a range of receptive field locations surrounding all four inducers. We found that V4 neurons exhibited increased responses for subjective stimuli compared to non-illusory controls in both monkeys. This effect was strongly enhanced if the central (illusory) surface of the subjective shape coincided with the center of the neuron’s receptive field. These results suggest that neurons in area V4 might play a crucial role in integrating visual cues to perceptually complete objects.