Receiving information The light energy that comes from the rays bouncing off of an object is converted into chemical energy by the cells in the
retina of the eye. This chemical energy is then converted into action potentials that are transferred through the
optic nerve and across the
optic chiasm, where it is first processed by the
lateral geniculate nucleus of the thalamus. From there the information is sent to the
primary visual cortex, region V1. It then travels from the visual areas in the
occipital lobe to the parietal and temporal lobes via two distinct anatomical streams. These two cortical visual systems were classified by Ungerleider and Mishkin (1982, see
two-streams hypothesis). One stream travels ventrally to the inferior temporal cortex (from V1 to V2 then through V4 to ITC) while the other travels dorsally to the posterior parietal cortex. They are labeled the "what" and "where" streams, respectively. The Inferior Temporal Cortex receives information from the
ventral stream, understandably so, as it is known to be a region essential in recognizing patterns, faces, and objects. (green) and
ventral stream (purple) originating in the primary visual cortex.
Single-cell function in the inferior temporal gyrus The understanding at the single-cell level of the IT cortex and its role of utilizing memory to identify objects and or process the visual field based on color and form visual information is a relatively recent in neuroscience. Early research indicated that the cellular connections of the temporal lobe to other memory associated areas of the brain – namely the
hippocampus, the
amygdala, the
prefrontal cortex, among others. These cellular connections have recently been found to explain unique elements of memory, suggesting that unique single-cells can be linked to specific unique types and even specific memories. Research into the single-cell understanding of the IT cortex reveals many compelling characteristics of these cells: single-cells with similar selectivity of memory are clustered together across the cortical layers of the IT cortex; the temporal lobe neurons have recently been shown to display learning behaviors and possibly relate to
long-term memory; and, cortical memory within the IT cortex is likely to be enhanced over time thanks to the influence of the afferent-neurons of the medial-temporal region. Further research of the single-cells of the IT cortex suggests that these cells not only have a direct link to the visual system pathway but also are deliberate in the visual stimuli they respond to: in certain cases, the single-cell IT cortex neurons do not initiate responses when spots or slits, namely simple visual stimuli, are present in the visual field; however, when complicated objects are put in place, this initiates a response in the single-cell neurons of the IT cortex. This provides evidence that not only are the single-cell neurons of the IT cortex related in having a unique specific response to visual stimuli but rather that each individual single-cell neuron has a specific response to a specific stimuli. The same study also reveals how the magnitude of the response of these single-cell neurons of the IT cortex do not change due to color and size but are only influenced by the shape. This led to even more interesting observations where specific IT neurons have been linked to the recognition of faces and hands. This is very interesting as to the possibility of relating to neurological disorders of prosopagnosia and explaining the complexity and interest in the human hand. Additional research from this study goes into more depth on the role of "face neurons" and "hand neurons" involved in the IT cortex. The significance of the single-cell function in the IT cortex is that it is another pathway in addition to the lateral geniculate pathway that processes most visual system: this raises questions about how does it benefit our visual information processing in addition to normal visual pathways and what other functional units are involved in additional visual information processing.
Information processing The information for color and form comes from P-cells that receive their information mainly from
cones, so they are sensitive to differences in form and color, as opposed to the M-cells that receive information about motion mainly from
rods. The neurons in the inferior temporal cortex, also called the inferior temporal visual association cortex, process this information from the P-cells. The
neurons in the ITC have several unique properties that offer an explanation as to why this area is essential in recognizing patterns. They only respond to visual stimuli and their receptive fields always include the
fovea, which is one of the densest areas of the retina and is responsible for acute central vision. These
receptive fields tend to be larger than those in the
striate cortex and often extend across the midline to unite the two visual half fields for the first time. IT neurons are selective for shape and/or color of stimulus and are usually more responsive to complex shapes as opposed to simple ones. A small percentage of them are selective for specific parts of the face. Faces and likely other complex shapes are seemingly coded by a sequence of activity across a group of cells, and IT cells can display both short or long-term memory for visual stimuli based on experience.
Object recognition There are a number of regions that work together within the ITC for processing and recognizing the information of "what" something is. In fact, discrete categories of objects are even associated with different regions. • The
fusiform gyrus or
fusiform face area (FFA) deals more with facial and body recognition rather than objects. • The
parahippocampal place area (PPA) helps differentiate between scenes and objects. • The
extrastriate body area (EBA) helps differentiate body parts from other objects. • And the lateral occipital complex (LOC) is used to determine shapes vs. scrambled stimuli. These areas must all work together, as well as with the
hippocampus, in order to create an array of understanding of the physical world. The hippocampus is key for storing the memory of what an object is/what it looks like for future use so that it can be compared and contrasted with other objects. Correctly being able to recognize an object is highly dependent on this organized network of brain areas that process, share, and store information. In a study by Denys et al., functional magnetic resonance imaging (
fMRI) was used to compare the processing of visual shape between humans and macaques. They found, amongst other things, that there was a degree of overlap between shape and motion sensitive regions of the cortex, but that the overlap was more distinct in humans. This would suggest that the human brain is better evolved for a high level of functioning in a distinct, three-dimensional, visual world. ==Clinical significance==