Orientation domain diversity in macaque area V2.

Single orientation domains in primary (V1) and second (V2) visual cortical areas are known to encode the orientation of visual contours. However, the visual world contains multiple and complex contour types. How do these domains handle such complexity? Using optical imaging methods, we have examined orientation response to two types of contours: real (luminance-defined) and illusory (inferred). We find that, unlike area V1, there are multiple types of orientation domain in V2. These include "real only" domains, "higher-order" domains (which respond to an orientation whether real or illusory), and other domains with nonmatching real/illusory orientation preference. We suggest that this plurality of orientation domains in V2 enables the complexities of local and global contour extraction.

Clustered cortical organization and the enhanced probability of intra-areal functional integration.

Similarly responsive neurons organize into submillimeter-sized clusters (domains) across many neocortical areas, notably in Areas V1 and V2 of primate visual cortex. While this clustered organization may arise from wiring minimization or from self-organizing development, it could potentially support important neural computation benefits. Here, we suggest that domain organization offers an efficient computational mechanism for intra-areal functional integration in certain cortical areas and hypothesize that domain proximity could support a higher-than-expected spatial correlation of their respective terminals yielding higher probabilities of integration of differing domain preferences. To investigate this hypothesis we devised a spatial model inspired by known parameters of V2 functional organization, where neighboring domains prefer either colored or oriented stimuli. Preference-selective joint probabilities were calculated for model terminal co-occurrence with configurations encompassing diverse domain proximity, shape, and projection. Compared to random distributions, paired neighboring domains (< or =1200 microm apart) yielded significantly enhanced coincidence of terminals converging from each domain. Using this reference data, a second larger-scale model indicated that V2 domain organization may accommodate relatively complete sets of intra-areal color/orientation integrations. Together, these data indicate that domain organization could support significant and efficient intra-areal integration of different preferences and suggest further experiments investigating prevalence and mechanisms of domain-mediated intra-areal integration.

A functional circuitry for edge-induced brightness perception.

The identification of visual contours and surfaces is central to visual scene segmentation. One view of image construction argues that object contours are first identified and then surfaces are filled in. Although there are psychophysical and single-unit data to suggest that the filling-in view is correct, the underlying circuitry is unknown. Here we examine specific spike-timing relationships between border and surface responses in cat visual cortical areas 17 and 18. With both real and illusory (Cornsweet) brightness contrast stimuli, we found a border-to-surface shift in the relative timing of spike activity. This shift was absent when borders were absent and could be reversed with relocation of the stimulus border, indicating that the direction of information flow is highly dependent on stimulus conditions. Furthermore, this effect was seen predominantly in 17-18, and not 17-17, interactions. These results demonstrate a border-to-surface mechanism at early stages of visual processing and emphasize the importance of interareal circuitry in vision.

Weakly modulated spike trains: significance, precision, and correction for sample size.

Many single-unit electrophysiological studies of visual cortex have investigated strong evoked responses to simple stimuli such as oriented gratings. Experiments involving other types of stimuli, such as natural scenes, higher-order features, and surface brightness, produce single-unit responses that are more difficult to interpret. Experiments with brightness, in particular, evoke single-unit responses that are typically weakly modulated. When the brightness is generated by a visual illusion such as the Cornsweet illusion, statistical tests are often necessary to distinguish true responses from baseline fluctuations. Here, using data collected from cat Areas 17 and 18 in response to real and illusory brightness stimuli, we provide a method for detecting and quantifying weak but significant periodic responses. By randomizing spike trains (via bootstrap methods), we provide confidence levels for response significance, permitting the evaluation of both weak and strong responses. We show that because of a strong dependence on total spike number, response significance can only be appropriately determined with randomized spike trains of similar spike number. Such randomizations can be performed for both stimulus-elicited and spontaneously occurring spike trains. By developing a method for generating randomized modulated spike trains (phase-restricted randomization) from actual recordings, we calculate upper and lower confidence limits of modulated spike trains and describe how measurement precision varies as a function of total spike count. Finally, using this randomization method, we describe how a correction function can be determined to correct for measurement bias introduced at low spike counts. These methods may also be useful in the study of small but potentially significant responses in other systems.

A chamber and artificial dura method for long-term optical imaging in the monkey.

Optical imaging over extended periods of time in non-human primates presents serious challenges because the dura mater must be removed to expose the cortical surface. We present a novel nylon imaging chamber with a transparent artificial dura implant, which allows repeated, long-term optical recordings from the cortex. The cylinder of the chamber is inserted into a cranial trephination and held in place with a minimum of screws and acrylic cement. A round patch of artificial dura with a perpendicular wall protects the cortical surface and slows re-growth of dural tissue within the chamber. A cap, manufactured from the same material as the cylinder, is screwed into the chamber and seals it completely. Over a period of 1-4 months, the chamber required a minimum of maintenance and stayed infection-free without local antibiotic application. We repeatedly performed optical imaging in the same animal with the advantages of shortened preparation time. To permit precise alignment and comparison of maps obtained from different imaging sessions, we developed a program that calculated a 2-dimensional spatial transformation between maps of different magnifications, translations, and distortions. We suggest that these methods provide a practical solution to long-term optical imaging in the anesthetized or alert monkey. The exclusive use of non-metallic materials offers the benefit of a lighter and more compact implant, and the possibility to perform MRI scans after chamber implantation.