Occipital and Temporal Lobes of the Cortex

Organization Principles of the Visual Cortex

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

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

1. Lateral geniculate nucleus (LGN) neurons: Serve 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 defining characteristic is orientation selectivity. These cells respond only to lines of a strictly defined orientation in space.
3. "Complex" and "hypercomplex" cells: Integrate excitation from an entire pool of "simple" neurons, enabling them to respond to much more complex and higher-level visual patterns.

Additionally, the visual cortex heavily utilizes binocular interaction. Experiments have proven that when light stimulates both eyes simultaneously, the vast majority—up to 80%—of visual cortex neurons become active.

Functional Areas: Areas 17, 18, and 19

Within the occipital lobe, specialized cytoarchitectonic areas are distinguished, each performing a specific part of visual image decoding.

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

However, their functional roles are strictly divided:

• Area 17: Specializes in the detailed analysis of static objects. During processing, it transmits information regarding the horizontal component of the visual image to area 18.
• Area 18: Unlike neighboring areas (17 and 19), it contains unique neurons sensitive to the degree of illumination. The primary presumed function of this zone is the analysis of moving targets. It sends data back to area 17 regarding the vertical component of the visible image.

When studying these structures, it is important to remember the property of plasticity. Receptive fields of cortical neurons (like the CNS as a whole) are not rigidly stable and hardwired once and for all. Under certain conditions, they are capable of undergoing significant modification 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 system, the basic structural and functional unit here is the neuronal "column."

The principal organizational feature of the auditory cortex is tonotopic organization. This is a strict spatial distribution of frequencies across the cortex, ensuring a detailed and comprehensive acoustic analysis of complex sounds.

Auditory columns are divided into three functional types based on their preferred stimulus:

• Frequency-specific columns: Their neurons are excited in response to a single specific sound frequency.
• Summation columns: Activated predominantly by binaural stimuli, meaning when a sound stimulus reaches both ears simultaneously.
• Suppression columns: Respond preferentially to monaural stimuli, when only one ear is stimulated.

Important note: Both visual and auditory projection areas of the cortex do not operate in complete isolation. Both zones additionally receive robust signals from non-specific thalamic nuclei, which modulates their level of excitability.

Neuronal Activity During Goal-Directed Behavior

The activity of visual and auditory cortex neurons is not limited to passive stimulus registration. Single-cell recording studies (particularly in the visual cortex of the cat brain) demonstrate a close relationship with ongoing behavioral activity.

When an animal performs a goal-directed behavioral act—such as food-acquisition behavior—the impulse firing of a cell reflects not only the physical parameters of light, but also the engagement of the visual cortex in the brain's overall integrative activity to achieve a useful outcome.