Hypercomplex cell

Initially discovered by David Hubel and Torsten Wiesel in 1965, hypercomplex cells are defined by the property of end-stopping, which is a decrease in firing strength with increasingly larger stimuli.

Ultimately, hypercomplex cells can provide a means for the brain to visually perceive corners and curves in the environment by identifying the ends of a given stimulus .

Investigations into the localization of function as well as the advent of single-cell recordings of neurons fostered greater insights into the processing of information from sensation to perception.

With reference to vision, Stephen Kuffler discovered areas of the retina, termed receptive fields, that upon stimulation, would influence the firing of ganglion cells.

[2] Two doctoral students in Kuffler’s lab at Johns Hopkins University, David Hubel and Torsten Wiesel, were tasked with extending his work from retinal ganglion cells to the visual cortex.

Serendipitously, Hubel and Wiesel had discovered that the cell was not responding to spots but to edges, namely the slide’s shadow as it was placed into the projector.

Following their initial finding, Hubel and Wiesel discovered the presence of a variety of visual processing cells, each with unique receptive field properties.

Furthermore, the regions exhibit mutual cancellation (antagonism) and produce stronger responses as the stimuli fill more space (spatial summation).

The motion selectivity of complex cells means that a response is elicited over a vast range of stimulus positions.

Accordingly, successive stimulations that proceed across the complex receptive field are required to elicit a sustained response; thereby, producing motion selectivity.

[4] Although the above definitions, established by Hubel and Wiesel, are the most widely accepted, some of their contemporaries had initially distinguished the classes along different criteria.

In sum, Hubel and Wiesel identified simple cells by discernibly separate excitatory and inhibitory regions that responded to stationary stimuli.

Furthermore, much like the subordinate processing cells, increasing illumination in a particular region elicited stronger responses (i.e. spatial summation).

In this case, a stimulus that extends too far in either direction (e.g. too far left or too far right) will begin to stimulate the antagonistic region and reduce the strength of the cell’s signal.

In 1968, Geoffrey Henry and Bogdan Dreher discovered simple and complex cells in Brodmann area 17 that exhibited end-stopping properties.

[2] Only a few years later, Charles Gilbert, a graduate student of Hubel and Wiesel, had confirmed end-stopping in the primary visual cortex.

[11] In his Nobel Prize lecture, Hubel explained that the hierarchy of visual processing cells proved to be more complicated and amorphous than initially believed, noting that the topic began to resemble a “jungle”.

An end-stopped cell would not respond to an edge on the side of the square because the line would stimulate both the activating and antagonistic regions simultaneously.

Although perceiving a square involves much more than the contributions of simple and complex cells, this example illustrates that the edges and borders of a stimulus (without input from the interior) are sufficient to interpret its form.

For example, the small-target motion detectors (STMDs) of many insects select for small moving targets but are inhibited or unresponsive to larger stimuli.

Brodmann area 17 (red) and higher order visual areas, Brodmann area 18 (orange) and Brodmann area 19 (yellow), are part of the visual cortex.
On-centre and off-surround presented alongside off-centre and off-surround retinal ganglion receptive fields
Cells with on-centre receptive fields fire when the excitatory centre is illuminated and are inhibited when the surround is illuminated. Off-centre cells respond to the opposite pattern of light.
Simple cells are sensitive to the orientation of a visual stimulus. A simple cell will fire weakly or not at all if both excitatory and inhibitory regions are activated (a), but will fire optimally if the stimulus is oriented within the excitatory region only (b). Orientation selectivity is produced by multiple centre-surround receptive fields aligned at a certain angle (c). A complex cell responds to moving stimuli and is sensitive to direction as well as orientation (d).
The hypercomplex cell above is stopped at one end (i.e. the right). As the length of the stimulus increases, it enters the antagonistic region, and causes a decrease in response (depicted as single-cell recording signals on the right). Note this cell is also sensitive to orientation, motion, and direction.
Top: End-stopped cells can detect curves. Note the properly oriented curve lies within the activating region but recedes and rotates before it enters the antagonistic regions. This cell is stopped at both ends and will not respond to lines that are not oriented 180˚. Bottom: End-stopped cells, like those that are stopped at one end, can also detect corners. The response of the cell will be stronger when the corner is only in the activating region (left image) and weaker when the corner enters the antagonistic region (right image).