Abstract
Inferior temporal (IT) cortex as the final stage of the ventral visual pathway is involved in visual object recognition. In our everyday life we need to recognize visual objects that are degraded by noise. Psychophysical studies have shown that the accuracy and speed of the object recognition decreases as the amount of visual noise increases. However, the neural representation of ambiguous visual objects and the underlying neural mechanisms of such changes in the behavior are not known. Here, by recording the neuronal spiking activity of macaque monkeys’ IT we explored the relationship between stimulus ambiguity and the IT neural activity. We found smaller amplitude, later onset, earlier offset and shorter duration of the response as visual ambiguity increased. All of these modulations were gradual and correlated with the level of stimulus ambiguity. We found that while category selectivity of IT neurons decreased with noise, it was preserved for a large extent of visual ambiguity. This noise tolerance for category selectivity in IT was lost at 60% noise level. Interestingly, while the response of the IT neurons to visual stimuli at 60% noise level was significantly larger than their baseline activity and full (100%) noise, it was not category selective anymore. The latter finding shows a neural representation that signals the presence of visual stimulus without signaling what it is. In general these findings, in the context of a drift diffusion model, explain the neural mechanisms of perceptual accuracy and speed changes in the process of recognizing ambiguous objects.
Highlights
Inferior temporal (IT) cortex, as the last stage in the ventral visual pathway, contains neurons that selectively respond to complex visual objects such as faces and bodies [1,2,3,4]
We analyzed the body category selective units of IT to address four questions: 1) What is the relationship between the level of stimulus ambiguity and the response amplitude? 2) What is the effect of noise on the temporal dynamic of the neural responses? 3) What is the relationship between the category selectivity and various levels of noise? 4) What is the neural mechanism of decreased accuracy and speed in recognizing ambiguous stimuli? To answer the last question we present our results in the context of a drift diffusion model of decision making
We investigated the effect of visual ambiguity on the IT neural responses by recording the spiking activities of 66 multiple units (MU) from the IT cortex of two macaque monkeys while they passively viewed noisy visual stimuli with various degrees of noise (10%, 30%, 45%, 60% and 100%) (Figure 1)
Summary
Inferior temporal (IT) cortex, as the last stage in the ventral visual pathway, contains neurons that selectively respond to complex visual objects such as faces and bodies [1,2,3,4]. While the level of the tolerance of IT neurons to such variations in the visual stimuli has been extensively explored [3,4,6,7,8,9,10,11,12], the effect of ‘ambiguity’ of the visual objects on IT neural responses is not clear yet. Driving in heavy rain or snow is an example of a situation where we need to recognize degraded visual objects such as pedestrians through windshield covered with snow or raindrops. Often, recognition of these ambiguous objects needs to be done as fast and accurately as possible to take the appropriate action. Psychophysical studies have shown that the accuracy and speed of the object recognition decreases as the stimulus ambiguity increases [5,13,14]
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