A hypercomplex cell (currently called an end-stopped cell) is a type of visual processing neuron in the mammalian cerebral cortex. 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. The sensitivity to stimulus length is accompanied by selectivity for the specific orientation, motion, and direction of stimuli. For example, a hypercomplex cell may only respond to a line at 45˚ that travels upward. Elongating the line would result in a proportionately weaker response. 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 .
Hypercomplex cells were originally characterized as the superordinate class of visual processing cells above complex and simple cells. Whereas complex cells were sensitive to moving stimuli of specific orientations that travel in a specific direction, simple cells only responded to properly oriented linear stimuli. Neither simple nor complex cells were believed to display end-stopping. Likewise, end-stopping was believed to be restricted to higher order visual areas (Brodmann area 18 and Brodmann area 19), but was later discovered to also exist in the primary visual cortex (Brodmann area 17). By 1968, Geoffrey Henry and Bogdan Dreher discovered simple and complex cells with end-stopping properties. Subsequently, hypercomplex cells were no longer recognized as a distinct class but rather a subtype of simple and complex cells. Currently, simple end-stopped and complex end-stopped cells are the terms of choice to describe neurons with end-stopping properties.
Background
Knowledge of cortical function was relatively limited by the 1950s. However, towards the end of the decade, the platform for understanding the cortex was being laid out. 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. These fields comprised two concentric layers, one excitatory and the other inhibitory. One type of receptive field was described as on-centre, containing an excitatory centre and an inhibitory surround, while the other type was termed off-centre, containing an inhibitory centre and an excitatory surround. Similar receptive fields were discovered in the lateral geniculate nucleus (LGN). 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. Hubel and Wiesel began recording cells in the cortex while presenting spots of light as stimuli. To start, the two had failed to produce any promising recordings, as the cells would not respond to the given stimuli. However, while inserting the glass slide into the projector, a strong signal was immediately elicited. 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. Hubel and Wiesel would later call this cell a complex cell, incorporating it into a hierarchy of subsequently discovered visual processing cells, which included the centre-surround, simple, complex, and hypercomplex cells (distinguishable by receptive fields)
Simple cells Following their initial finding, Hubel and Wiesel discovered the presence of a variety of visual processing cells, each with unique receptive field properties. At the lowest and simplest level of the hierarchy are the aforementioned centre-surround cells of the retinal ganglion and LGN. Next, within the visual cortex, are simple cells. Simple cells exist within the primary visual cortex (Brodmann Area 17). These cells are found specifically in layer IV, at which most outgoing projections from the LGN terminate. The receptive fields of simple cells are non-concentric and linear, in which excitatory and inhibitory regions exist adjacent to one another. Thus, a response is elicited by stationary linear stimuli. Furthermore, the regions exhibit mutual cancellation (antagonism) and produce stronger responses as the stimuli fill more space (spatial summation). A discerning feature of simple cells is that their responses display orientation and positional selectivity. This means that a simple cell fires at an optimal orientation. Elicited responses get progressively weaker as a stimulus's orientation shifts sub-optimally and ceases to fire when at 90˚ from the optimal orientation. Positional selectivity simply refers to the cell's receptiveness to the position of the stimulus within part or all of the excitatory/inhibitory regions. Accordingly, simple cell receptive fields exist in a variety of different geometries and sizes for all possible orientation and positions in the visual field. It is presumed that multiple concentric LGN receptive fields converge in a line to develop a single simple receptive field.
… excerpt ends here. Continue reading the full article.
