Inferior temporal cortex
Inferior Temporal (IT) Cortex is the cerebral cortex on the inferior convexity of the temporal lobe in primates including humans. It is crucial for visual object recognition and is considered to be the final stage in the ventral cortical visual system. It corresponds to cytoarchitecture Areas 20 and 21 (in Brodmans’s terminology) and Area TE (in von Economo’s). In humans it consists of the middle and inferior temporal gyri. (See Figure 1 and Figure 2)
History of the discovery of the role of IT cortex in vision
Kluver and Bucy (1938) showed that bilateral removal of the temporal lobes in monkeys produced a constellation of strange symptoms including docility, mouthing uneatable objects, indiscriminate sexuality and the inability to recognize objects visually that became known as the Kluver-Bucy syndrome. Pribram and Mishkin then fractionated this syndrome and showed that the visual recognition deficit and only this aspect of the syndrome followed lesions of the inferior temporal cortex. The other aspects of the syndrome were associated with damage to the amygdaloid nucleus.
Effects of IT lesions
In monkeys IT lesions produce a severe and permanent impairment in learning and remembering to recognize visual stimuli although recognition in other modalities is normal and there are no visuo-sensory losses sufficient to explain the visual recognition deficit. Thus, IT lesions result in visual agnosia.
In humans, impairments on visual recognition are often found after bilateral or right hemisphere damage to IT cortex. Sometimes the deficit is a general impairment in visual recognition or visual agnosia; more rarely it can be a specific difficulty in recognizing faces (prosopagnosia), colors (achromatopsia) or even a category specific agnosia such as difficulty in recognizing animals or man-made objects.
Anatomical bases of the visual functions of IT cortex
The visual functions of IT cortex depend on input it receives from striate cortex (primary visual cortex of V1) over a cortico-cortical route that includes the splenium of the corpus callosum and the anterior commissure and has synapses on Areas V2, V4 and TEO. The projection from a large thalamic nucleus, the inferior pulvinar, to IT cortex may play a role in visual attention.
Properties of IT neurons
Individual IT neurons have several properties that help explain the crucial role of this area in pattern recognition:
- IT cells only respond to visual stimuli.
- The receptive fields of IT neurons always include the fovea or center of gaze, that is, the part of the retina most involved in pattern recognition.
- The receptive fields of IT neurons tend to be large, much larger than in striate cortex, affording the opportunity for stimulus generalization within the receptive field.
- The receptive fields of IT neurons often extend across the midline into both visual half fields thus uniting the two halves of space for the first time. This property depends on Interhemispheric connections by way of the splenium and anterior commissure.
- IT neurons are usually selective for the shape or color of the stimulus or both parameters and almost all respond more to complex than simple shapes.
- A small percentage of IT units are selective for facial images. Some are sensitive to emotional expression and some to direction of eye gaze. Face selective cells are much more common in the superior temporal sulcus. Infant monkeys as well as adults have face selective cells. Cells selective for hands are also found.
- Faces and probably other shapes appear to be coded by the pattern of activity across an ensemble of cells rather than by the dedicated firing of a grandmother cell, that is, a cell that only fires to a highly specific visual percept such as one’s grandmother.
- The selectivity of IT cells for shape is usually invariant over changes in stimulus size, contrast, color and exact location on the retina.
- There appears to be a vertical organization to the stimulus selectivity of IT neurons.
- The activity of IT neurons can be modulated by the animal’s attention.
- IT cells can show both short or long term memory for visual stimuli and their selectivity can be modified by experience.
Functional magnetic imaging of IT cortex
In humans, functional magnetic imaging (fMRI) shows that visual objects activate IT cortex. In addition, there are restricted regions of IT cortex that are selectively activated by specific classes of stimuli, for example faces in a region of the fusiform gyrus (fusiform face area or FFA), eye position in a dorsal and anterior region, visual scenes in part of the hippocampal gyrus (parahippocampal place area or PPA) and body parts in a region of the fusiform gyrus (fusiform body area or FBA).
In macaque IT there are also specific regions activated by faces and virtually all the neurons recorded in these areas are selective for faces. There appears to be a body part area.
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