The striate cortex, much like the LGN, possesses a retinotopic map. The representation of the macula is located in the posterior and largest part of both gyri, with progressively more peripheral areas of the retina projecting to the anterior regions of these gyri. This results in a systematic mapping of retinal loci across the surface of the striate cortex. The geniculostriate pathway primarily terminates in layer 4 of the striate cortex, while the projection from the intralaminar LGN ends in blobs within layers 2 and 3. Within layer 4C, axons representing each eye terminate in alternate adjacent patches, defining ocular dominance columns. Neurons within these columns preferentially respond to input from one eye, whereas near the border between two ocular dominance columns, neurons respond equally to input from both eyes. Apart from monocular cells in layer 4C, receptive fields of neurons in the striate cortex are more complex than those in the LGN. Neurons in other layers are often binocular and respond to stimulation from both eyes, although one eye's input tends to dominate. Additionally, cortical neurons outside layer 4C exhibit orientation selectivity and respond optimally to stimuli such as bars or edges positioned and oriented in a particular manner. These neurons, known as "simple cells," appear to receive input from cells with concentric center-surround receptive fields, aligned with "on" centers in a row flanked by antagonistic regions. "Complex" cortical neurons, similar to "simple" cells, also display orientation specificity but respond optimally to a specific stimulus orientation anywhere in their receptive field, without the presence of flanking excitatory and inhibitory zones. Complex cells may also exhibit direction selectivity, responding to stimuli moving in one direction but not in the opposite direction. The receptive field of a complex cell can be considered as a combination of adjacent simple cells with similar orientation selectivity. Due to the uniform orientation selectivity of neurons within a specific cortical zone, they form orientation columns.
Like the LGN, the striate cortex contains a retinotopic map. The representation of the macula occupies the most posterior and largest part of both gyri, and progressively more peripheral areas of the retina are projected to more anterior parts of these gyri. Overall, there is an orderly mapping of retinal loci across the surface of the striate cortex (see
Fig. 8.10
The geniculostriate pathway ends chiefly in layer 4 of the striate cortex (
Fig. 8.11 ), whereas the projection from the intralaminar LGN terminates in so-called blobs in layers 2 and 3. Similarly, axons that represent one eye or the other terminate within layer 4C in alternate adjacent patches that define ocular dominance columns. Cortical neurons in such a column respond preferentially to input from one eye. Near the border between two ocular dominance columns, neurons respond about equally to input from the two eyes.
The receptive fields of neurons in the striate cortex, aside from the monocular cells in layer 4C, are more complex than those of LGN neurons. Neurons in other layers may be binocular and respond to stimulation of both eyes, although the input from one eye often dominates (see ). In addition, cortical neurons outside layer 4C often show orientation selectivity (i.e., they respond best when the stimulus, such as a bar or an edge, is oriented and positioned in a particular way;
Fig. 8.12 ). These “simple cells” appear to be responding as though they received input from cells whose concentric center-surround receptive fields were arranged in such a way that their “on” centers were aligned in a row flanked by antagonistic regions. “Complex” cortical •Fig. 8.10 Theleftvisualfieldisrelayed(viatheLGNandvisualradiation)totheprimaryvisualcortexoftherighthemisphere,asapoint-to-pointretinotopicmap.Therepresentationofeachpartofvisualspaceisproportionaltothenumberofafferentaxonswithreceptivefieldsinthatpartofspace.Asaresult,theareaofmacularrepresentation(neartheoccipitalpole)islargerthanthatfortherestofthebinocularandmonocularfields.Notethatthelowerhalfofthefieldisrepresentedinthecuneusgyrusabovethecalcarinesulcusandtheupperhalfofthefieldinthelingualgyrusbelowthesulcus.(RedrawnfromPurvesD,etal[eds].Neuroscience. 3rded.Sunderland,MA:Sinauer;2004.) •Fig. 8.11 DiagramofvisualinformationflowintothevisualcortexfromtheLGNanditsprojectiontotheextrastriatecortex,tothesuperiorcolliculus(SC),andbacktotheLGN.M,magnocellularpath;P,parvocellularpath.(RedrawnfromSquireLR,etal[eds].Fundamental Neuroscience. SanDiego,CA:AcademicPress;2002.)LGN m cell LGN p cell LGN intralaminar p cell Cortex Cortex SC LGN 4B 2+3 5 6 4C˜4C°Blobs (2+3) neurons are similar to “simple” cells in that they are orientation specific, but instead of having flanking excitatory and inhibitory zones, they respond best to a particular stimulus orientation anywhere in their receptive field. They may also display direction selectivity; that is, they may respond when the stimulus is moved in one direction but not when it is moved in the opposite direction (see
Fig. 8.12 ). The receptive field of a “complex” cell may be thought of as a composite of adjacent “simple” cells with similar orientation selectivity. Because such neurons in a particular zone of the cortex all tend to have the same orientation selectivity, they are considered to form an orientation column (Fig. 8.13