Occipital and Temporal Lobes of the Cortex

Principles of visual cortex organization

The occipital cortex is the main center for cortical processing of visual stimuli. According to the foundational works of D. Hubel and T. Wiesel (1977), its architecture is based on a columnar organization. This means that nerve cells with identical receptive field characteristics are not scattered randomly, but arranged into distinct vertical structures—«columns».

The key mechanism underlying the properties of such cells is the principle of convergence. The receptive field of a cortical neuron is formed because excitatory impulses from multiple cells in lower divisions of the central nervous system converge upon it. Signal processing follows a strict hierarchy:

1. Neurons of the lateral geniculate body (LGB): act as the primary subcortical relay and possess simple circular receptive fields.
2. «Simple» cortical neurons: receive converging signals from a group of LGN cells. Their main distinguishing feature is orientation selectivity. These cells respond only to lines of a strictly defined orientation in space.
3. «Complex» and «hypercomplex» cells: gather excitation from an entire pool of «simple» neurons, allowing them to respond to much more complex and high-level visual images.

In addition, the visual cortex actively utilizes binocular interaction. It has been experimentally proven that during light stimulation of both eyes simultaneously, the vast majority—up to 80%—of visual cortex neurons are activated.

Functional areas: fields 17, 18, and 19

Within the occipital lobe, specialized regions are distinguished—cytoarchitectonic fields, each performing a specific part of the work in decoding the visual image.

Both key fields (17 and 18) receive afferent nerve fibers predominantly from the layers of the dorsal nucleus of the lateral geniculate body. They are characterized by a retinotopic projection, meaning that absolutely the entire visual field is spatially and orderly represented in the cortex of these fields.

However, their functional tasks are strictly divided:

• Field 17: specializes in the detailed analysis of static objects. During processing, it transmits information about the horizontal component of the visual image to field 18.
• Field 18: unlike neighboring fields (17 and 19), contains unique neurons sensitive to the illumination level. The main presumed function of this zone is the analysis of moving targets. It sends data regarding the vertical component of the visible image back to field 17.

When studying these structures, it is important to remember the property of plasticity. The receptive fields of cortical neurons (as well as the entire CNS) are not rigidly stable and fixed once and for all. Under certain conditions, they are capable of undergoing significant changes and restructuring their characteristics.

Auditory cortex of the temporal lobe

The temporal lobe assumes the crucial function of processing auditory information. As in the visual analyzer, the basic structural and functional unit here is the neuronal «column».

The main principle of auditory cortex organization is tonotopic organization. This is a strict spatial distribution of frequencies across the cortex that ensures a detailed and comprehensive acoustic analysis of complex sounds.

The auditory columns themselves are divided into three functional types depending on the preferred stimulus:

• Frequency-specific columns: neurons within them are excited in response to a sound of one specific frequency.
• Summation columns: are activated predominantly by a binaural stimulus, that is, when the sound stimulus enters both ears simultaneously.
• Suppression columns: respond predominantly to a monaural stimulus, when only one ear is stimulated.

Important addition: both the visual and auditory projection areas of the cortex do not work in complete isolation. Both of these zones additionally receive powerful signals from non-specific thalamic nuclei, which affects their level of excitability.

Neuronal activity during goal-directed behavior

The activity of visual and auditory cortex neurons is not limited to the passive registration of stimuli. Studies of single-cell spike activity (specifically, neurons in the visual cortex of the cat brain) demonstrate a close relationship with ongoing activity.

Under conditions where an animal performs a goal-directed behavioral act—for instance, during foraging behavior—the cell's impulse activity reflects not only the physical parameters of light, but also the involvement of the visual cortex in the general integrative activity of the brain to achieve a useful result.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Influence of non-specific thalamic nuclei on the activity of primary sensory cortical areas
• Mechanisms of orientation selectivity formation in simple cells
• Interaction of fields 17, 18, and 19 in forming a holistic visual image
• Biophysiological foundations of cortical receptive field plasticity
• Spike activity patterns of cortical neurons during foraging behavior

Other topics in the section "Special Physiology of the CNS"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.