Preferential encoding of visual categories in parietal cortex compared with prefrontal cortex

Abstract

The ability to recognize the behavioral significance, or category membership, of sensory stimuli is critical for interpreting the meaning of events in our environment. Prior neurophysiological studies of visual categorization found categorical representations of stimuli in prefrontal cortex (PFC), an area closely associated with cognitive and executive functions. Recent studies have also identified neuronal category signals in parietal areas typically associated with visual-spatial processing. It has been proposed that category-related signals in parietal cortex and other visual areas may result from “top-down” feedback from PFC. We directly compared neuronal activity in the lateral intraparietal (LIP) area and PFC in monkeys performing a visual motion categorization task. Here we show that LIP shows stronger, more reliable, and shorter latency category signals than PFC. This suggests that LIP is strongly involved in visual categorization, and argues against the idea that parietal category signals arise from feedback from PFC during this task.

Neuronal correlates of a visual “sense of number” in primate parietal and prefrontal cortices

“…This suggests that visual numerosity is extracted first in the termination zone of the dorsal visual pathway of the parietal cortex based on bottom-up process and later conveyed to the functionally connected lateral PFC (36). Even in trained animals, parietal signals of visual numerical categories do not arise as a result of feedback from PFC (22).…”

“…Such "number neurons" abstractly represent the number of items across space, time, and modalities (15,16,17). Number neurons have also been traced indirectly in the human brain using functional MRI (fMRI) (18,19).However, because neurons can be trained to represent behaviorally meaningful categories (20,21,22), it has been argued (23) that the presence of previously described number neurons in trained animals might be a product of intense learning, rather than a reflection of a spontaneous number sense. For the same reason, the coding scheme for numerosity has been debated (23): Is the spontaneous neuronal code for numerosity a summation code, as evidenced by monotonic discharges as a function of quantity (14,24), or a labeled-line code as witnessed by numerosity-selective neurons tuned to preferred numerosities analogous to those found in monkeys performing numerical tasks (25,26)?…”

Trial-to-trial, uncertainty-based adjustment of decision boundaries in visual categorization

“…First, we predict that populations of orientation-sensitive neurons in early visual cortex not only encode an estimate of orientation but also trialto-trial information about sensory uncertainty and that this encoded sensory uncertainty correlates with the animal's decision boundary as obtained from its behavior in the categorization task. Second, we predict that the activity of category-sensitive neurons, which might be found in prefrontal cortex (2) or lateral intraparietal cortex (LIP) (1,5), is linearly related to the logarithm of the posterior probability ratio over category. This prediction is consistent with patterns found in LIP, where category probability can be reconstructed from a logistic mapping on neural activity (33).…”

“…In this categorization problem, you are determining whether the image of the approaching woman falls into the narrow category of images of your friend or the wide category of images of all other people. Categorization can be modeled as a process of applying one or more decision boundaries to a noisy measurement in a space of stimulus features (1)(2)(3)(4)(5)(6)(7). The example suggests that adjusting such decision boundaries based on the current level of sensory uncertainty might be a better strategy than using uncertainty-independent decision boundaries.…”

Deep Hierarchies in the Primate Visual Cortex: What Can We Learn for Computer Vision?

“…In contrast, an explicit category representation is present in the prefrontal cortex [53] and surprisingly also in the posterior parietal cortex (area LIP, [52]). Category information even occurs earlier, is stronger and more reliable in parietal cortex than in the prefrontal cortex [168].…”