Visual response properties of single neurons in the temporal pole of behaving monkeys

Activation reduction in anterior temporal cortices during repeated recognition of faces of personal acquaintances

Repeated recognition of the face of a familiar individual is known to show semantic repetition priming effect. In this study, normal subjects were repeatedly presented faces of their colleagues, and the effect of repetition on the regional cerebral blood flow change was measured using positron emission tomography. They repeated a set of three tasks: the familiar-face detection (F) task, the facial direction discrimination (D) task, and the perceptual control (C) task. During five repetitions of the F task, familiar faces were presented six times from different views in a pseudorandom order. Activation reduction through the repetition of the F tasks was observed in the bilateral anterior (anterolateral to the polar region) temporal cortices which are suggested to be involved in the access to the long-term memory concerning people. The bilateral amygdala, the hypothalamus, and the medial frontal cortices, were constantly activated during the F tasks, and considered to be associated with the behavioral significance of the presented familiar faces. Constant activation was also observed in the bilateral occipitotemporal regions and fusiform gyri and the right medial temporal regions during perception of the faces, and in the left medial temporal regions during the facial familiarity detection task, which are consistent with the results of previous functional brain imaging studies. The results have provided further information about the functional segregation of the anterior temporal regions in face recognition and long-term memory.

The distinct modes of vision offered by feedforward and recurrent processing

An analysis of response latencies shows that when an image is presented to the visual system, neuronal activity is rapidly routed to a large number of visual areas. However, the activity of cortical neurons is not determined by this feedforward sweep alone. Horizontal connections within areas, and higher areas providing feedback, result in dynamic changes in tuning. The differences between feedforward and recurrent processing could prove pivotal in understanding the distinctions between attentive and pre-attentive vision as well as between conscious and unconscious vision. The feedforward sweep rapidly groups feature constellations that are hardwired in the visual brain, yet is probably incapable of yielding visual awareness; in many cases, recurrent processing is necessary before the features of an object are attentively grouped and the stimulus can enter consciousness.

Response differences in monkey TE and perirhinal cortex: stimulus association related to reward schedules

Anatomic and behavioral evidence shows that TE and perirhinal cortices are two directly connected but distinct inferior temporal areas. Despite this distinctness, physiological properties of neurons in these two areas generally have been similar with neurons in both areas showing selectivity for complex visual patterns and showing response modulations related to behavioral context in the sequential delayed match-to-sample (DMS) trials, attention, and stimulus familiarity. Here we identify physiological differences in the neuronal activity of these two areas. We recorded single neurons from area TE and perirhinal cortex while the monkeys performed a simple behavioral task using randomly interleaved visually cued reward schedules of one, two, or three DMS trials. The monkeys used the cue's relation to the reward schedule (indicated by the brightness) to adjust their behavioral performance. They performed most quickly and most accurately in trials in which reward was immediately forthcoming and progressively less well as more intermediate trials remained. Thus the monkeys appeared more motivated as they progressed through the trial schedule. Neurons in both TE and perirhinal cortex responded to both the visual cues related to the reward schedules and the stimulus patterns used in the DMS trials. As expected, neurons in both areas showed response selectivity to the DMS patterns, and significant, but small, modulations related to the behavioral context in the DMS trial. However, TE and perirhinal neurons showed strikingly different response properties. The latency distribution of perirhinal responses was centered 66 ms later than the distribution of TE responses, a larger difference than the 10-15 ms usually found in sequentially connected visual cortical areas. In TE, cue-related responses were related to the cue's brightness. In perirhinal cortex, cue-related responses were related to the trial schedules independently of the cue's brightness. For example, some perirhinal neurons responded in the first trial of any reward schedule including the one trial schedule, whereas other neurons failed to respond in the first trial but respond in the last trial of any schedule. The majority of perirhinal neurons had more complicated relations to the schedule. The cue-related activity of TE neurons is interpreted most parsimoniously as a response to the stimulus brightness, whereas the cue-related activity of perirhinal neurons is interpreted most parsimoniously as carrying associative information about the animal's progress through the reward schedule. Perirhinal cortex may be part of a system gauging the relation between work schedules and rewards.

Neurochemical gradients along monkey sensory cortical pathways: calbindin-immunoreactive pyramidal neurons in layers II and III

We examined the distribution of neurons containing immunoreactivity for three calcium-binding proteins, calbindin, parvalbumin and calretinin, as well as nonphosphorylated neurofilament protein, in cortical areas along the ventral and dorsal cortical visual pathways, and in ventrally-directed somatosensory and auditory cortical pathways. Calbindin-immunoreactive pyramidal neurons showed the most prominent regional differences. They were largely restricted to layers II and III and their number monotonically increased from the primary sensory areas to the anteroventral areas along the ventral visual pathway and along the ventrally-directed somatosensory and auditory pathways. The number of calbindin-immunoreactive pyramidal neurons in layers II and III also increased along the dorsal visual pathway, but the number in the last recognized stage of the dorsal visual pathway (area 7a) was significantly smaller than that at the corresponding stage in the ventral visual pathway (TE). The number of calbindin-immunoreactive pyramidal neurons was highest in layers II and III of areas 35/36, TG, and TF/TH, which represent terminal cortical regions of the pathways. These results show neurochemical differences between cortical areas located at early and late stages along serial corticocortical pathways, as well as confirming differences between pyramidal neurons in the supragranular and infragranular layers.

Global and fine information coded by single neurons in the temporal visual cortex

When we see a person's face, we can easily recognize their species, individual identity and emotional state. How does the brain represent such complex information? A substantial number of neurons in the macaque temporal cortex respond to faces. However, the neuronal mechanisms underlying the processing of complex information are not yet clear. Here we recorded the activity of single neurons in the temporal cortex of macaque monkeys while presenting visual stimuli consisting of geometric shapes, and monkey and human faces with various expressions. Information theory was used to investigate how well the neuronal responses could categorize the stimuli. We found that single neurons conveyed two different scales of facial information in their firing patterns, starting at different latencies. Global information, categorizing stimuli as monkey faces, human faces or shapes, was conveyed in the earliest part of the responses. Fine information about identity or expression was conveyed later, beginning on average 51 ms after global information. We speculate that global information could be used as a 'header' to prepare destination areas for receiving more detailed information.

Categorization of complex visual images by rhesus monkeys. Part 2: single-cell study

In order to investigate the neural coding of ordinate-level visual categories, single-cell recordings were made in the anterior temporal cortex of two rhesus monkeys performing a categorization of colour images of trees versus images of other objects. Neurons showed a high average degree of selectivity for these complex colour images. Although most neurons responded to trees and non-trees, about a quarter responded in a category-specific manner, e.g. to trees but not non-trees, and about one-tenth responded almost exclusively to exemplars of the trained category. The responses of these neurons were largely invariant for stimulus transformations, e. g. changes in position or size, and decreased with the degree of image scrambling, mimicking the behavioural results. However, the responses of single neurons were insufficiently stimulus invariant to accommodate the entire range of variability present in the features of exemplars within the same category. This strong within-category selectivity challenges the idea that a prototype is represented at the single neuron level, but suggests that ordinate-level categorization is based on a population of neurons, each selective for a limited set of exemplars.

Perirhinal cortex ablation impairs configural learning and paired-associate learning equally

Combined damage to the perirhinal and entorhinal cortex has been implicated in the formation of stimulus-stimulus associative memories. We show in this article that relative to three normal controls three cynomolgus monkeys with ablations restricted to the perirhinal cortex were impaired on a visual paired associate learning task in which subjects had to learn which of two visual stimuli were associated with a cue stimulus. The subjects with perirhinal cortex ablations also showed an impairment of a similar magnitude on a visual configural learning task in which they had to learn which of two configurations of visual stimuli were associated with food-reward. The stimuli in both tasks were comprised of alphanumeric characters presented upon a touch-screen. Both groups made fewer errors on the configural learning task than on the paired associate learning task. We suggest that performance on both tasks relies critically on the perirhinal cortex due to the specialization of the perirhinal cortex in processing knowledge about objects. We argue that the specializations of this system and of other memory systems such as the hippocampal-fornix spatial/episodic memory system, are conferred by the specialization of their anatomical connections to other structures. We reject the notion that there are specific memory processes such as the hippocampal based configural associative system that was proposed to be critical for configural associative learning.

Signal timing across the macaque visual system

The onset latencies of single-unit responses evoked by flashing visual stimuli were measured in the parvocellular (P) and magnocellular (M) layers of the dorsal lateral geniculate nucleus (LGNd) and in cortical visual areas V1, V2, V3, V4, middle temporal area (MT), medial superior temporal area (MST), and in the frontal eye field (FEF) in individual anesthetized monkeys. Identical procedures were carried out to assess latencies in each area, often in the same monkey, thereby permitting direct comparisons of timing across areas. This study presents the visual flash-evoked latencies for cells in areas where such data are common (V1 and V2), and are therefore a good standard, and also in areas where such data are sparse (LGNd M and P layers, MT, V4) or entirely lacking (V3, MST, and FEF in anesthetized preparation). Visual-evoked onset latencies were, on average, 17 ms shorter in the LGNd M layers than in the LGNd P layers. Visual responses occurred in V1 before any other cortical area. The next wave of activation occurred concurrently in areas V3, MT, MST, and FEF. Visual response latencies in areas V2 and V4 were progressively later and more broadly distributed. These differences in the time course of activation across the dorsal and ventral streams provide important temporal constraints on theories of visual processing.

The primate temporal pole: its putative role in object recognition and memory

In this article, we consider both the ventral temporopolar cortex and the perirhinal cortex (areas 35 and 36) as the anterior ventromedial temporal (aVMT) cortex, and discuss its role based on recent data in monkeys and human subjects. In monkeys, the aVMT cortex receives its primary input from area TE, and only minor input from other cortical areas. Laminar patterns of connections suggest that the aVMT cortex is a hierarchically higher-order area than area TE. Lesions of this cortex produce deficits in the learning and performance of visual memory tasks. Neurons in the aVMT cortex respond selectively to complex stimuli and changes in activity related to visual memory tasks. In humans, damage of this cortex induces deficits in the recognition of familiar objects and faces. The aVMT cortex is activated during recognition of familiar faces. In addition, the aVMT cortex is one of the most vulnerable areas in Alzheimer's disease. All these data indicate that the aVMT cortex is a higher-order visual cortical area that is related to object recognition and memory. The anterior area TE has been implicated in both functions. We propose here that these areas and the anterior entorhinal cortex are designated as the temporal pole, a brain region which is specialized for both object recognition and memory.

The anatomy, physiology and functions of the perirhinal cortex

The perirhinal cortex is a polymodal association area that contributes importantly to normal recognition memory. A convergence of recent findings from lesion and electrophysiological studies has provided new evidence that this area participates in an even broader range of memory functions than previously thought, including associative memory and emotional memory, as well as consolidation functions. These results are consistent with neuroanatomical research showing that this area has strong and reciprocal connections with widespread cortical sensory areas and with other memory-related structures, including the hippocampal formation and amygdala.