Question

To what features of the visual world do simple and complex cells of the mammalian visual cortex respond? How do these cells differ?

Short answer

Simple cells in the visual cortex respond to specific visual stimuli such as contrast of edges and line orientations, having defined 'on' and 'off' areas in their receptive fields. Complex cells, on the other hand, are sensitive to these stimuli no matter where they are placed in the receptive field, also responding to the direction of movement and having binocular disparity, unlike simple cells.

Step-by-step solution

- Describing the function of simple cells

Simple cells in the mammalian visual cortex respond to specific visual stimuli like the orientation of lines and the contrast of edges. They have specific receptive fields with clear 'on' and 'off' regions that respond directly to light and dark stimuli in their field.

- Discussing the function of complex cells

Complex cells, like simple cells, also respond to specific orientations of lines and the contrast of edges. However, they differ as they do not have 'on' and 'off' regions. Rather, they respond to a particular type of stimulus regardless of its exact location within the receptive field. They are more spatially invariant, are sensitive to the direction of movement of the stimulus, and to binocular stimulus.

- Highlighting the difference between simple and complex cells

Both simple and complex cells respond to orientation and edges, but they differ in two main ways. First, simple cells have defined 'on' and 'off' areas in their receptive fields, while complex cells don't. Secondly, complex cells react to the moving pattern of a stimulus across their entire receptive field and show binocular disparity, which is not the case for simple cells.

Key concepts

Simple Cells

The mammalian visual cortex harbors neurons referred to as simple cells, which exhibit a distinct sensitivity to particular visual cues. These neuronal cells specialize in detecting edges and variations in contrast, acting as initial processors for visual stimuli composed of lines and borders.

Simple cells have what are known as receptive fields, with set 'on' and 'off' zones. In these fields, certain areas are stimulated by light (the 'on' region), while adjacent regions are inhibited by light (the 'off' region). This creates a mechanism where simple cells respond optimally to lines of a specific orientation that align with their receptive field.

For example, when a vertical line passes through the 'on' region of a simple cell's receptive field, it triggers a reaction. However, if the line is horizontal and doesn't match the orientation of the receptive field, the cell responds weakly or not at all. This characteristic enables simple cells to construct a detailed map of the edges and contours within the visual scene.

Complex Cells

Complex cells, also residents of the mammalian visual cortex, take on a more advanced role compared to simple cells. They share the attribute of responding to certain line orientations and contrasts; however, their approach to processing visual information is more adaptable.

Unlike simple cells, complex cells do not possess clear-cut 'on' and 'off' regions within their receptive fields. This means that their response to a visual stimulus, such as a line or an edge, is less dependent on its precise location within the receptive field. Complex cells are capable of detecting a stimulus's orientation even if it moves across different areas of the field, showcasing spatial invariance.

Furthermore, complex cells are sensitive to the actual movement, or the direction and speed, of the visual stimuli. Their receptive fields integrate information over a wider spatial area, which provides them the ability to perceive movement within their field of view, a critical aspect of dynamic vision.

Receptive Fields

Understanding receptive fields is central to grasping how the visual system discerns and interprets diverse visual stimuli. These are specific regions of the sensory space (like the retina for vision) where the presence of a stimulus will alter the firing of a neuron.

A neuron's receptive field can often be visualized as a two-dimensional map, where some areas can be excitatory ('on' regions) and others inhibitory ('off' regions). In the context of visual processing, this design allows neurons to respond preferentially to specific spatial patterns like lines, edges, and contrasts positioned within those fields.

It's important to note that receptive fields vary in complexity and function across different neurons. Simple cell receptive fields are structured with distinct 'on' and 'off' areas, preferrably stimulating those cells when a stimulus aligns with its receptive field's preferred orientation. In contrast, complex cells have receptive fields that are less rigid in structure, focusing more on the properties like orientation and movement across a larger area.

Visual Stimulus Orientation

Orientation of a visual stimulus is a fundamental element to which the neurons in the mammalian visual cortex are tuned. The ability to detect and respond to lines and edges of different inclinations allows animals, including humans, to interpret the shape and structure of their environment.

Neurons, particularly simple cells, have receptive fields that are oriented in such a way that maximizes their response to lines and edges at specific angles. For instance, some neurons are highly responsive to vertical stimuli, while others may prefer horizontal or diagonal orientations. This selective sensitivity to orientation is essential, as it helps to detect boundaries and contours of objects within the visual field.

The orientation of a visual stimulus is a critical cue that the visual system uses for early feature detection, which is subsequently integrated into a more refined and coherent representation of the visual world in higher processing stages of the cortex.