Sean P. MacEvoy

Distances between Real-World Locations Are Represented in the Human Hippocampus

Spatial navigation is believed to be guided in part by reference to an internal map of the environment. We used functional magnetic resonance imaging (fMRI) to test for a key aspect of a cognitive map: preservation of real-world distance relationships. University students were scanned while viewing photographs of familiar campus landmarks. fMRI response levels in the left hippocampus corresponded to real-world distances between landmarks shown on successive trials, indicating that this region considered closer landmarks to be more representationally similar and more distant landmarks to be more representationally distinct. In contrast, posterior visually responsive regions such as retrosplenial complex and the parahippocampal place area were sensitive to landmark repetition and encoded landmark identity in their multivoxel activity patterns but did not show a distance-related response. These data suggest the existence of a map-like representation in the human medial temporal lobe that encodes the coordinates of familiar locations in large-scale, real-world environments.

Decoding the Representation of Multiple Simultaneous Objects in Human Occipitotemporal Cortex

Previous work using functional magnetic resonance imaging has shown that the identities of isolated objects viewed by human subjects can be extracted from distributed patterns of brain activity. Outside the laboratory, however, objects almost never appear in isolation; thus it is important to understand how multiple simultaneously occurring objects are encoded by the visual system. We used multivoxel pattern analysis to examine this issue, testing whether activity patterns in the lateral occipital complex (LOC) evoked by object pairs showed an ordered relationship to patterns evoked by their constituent objects. Applying a searchlight analysis to identify voxels with the highest signal-to-noise ratios, we found that responses to object pairs within these informative voxels were well predicted by the averages of responses to their constituent objects. Consistent with this result, we were able to classify object pairs by using synthetic patterns created by averaging single-object patterns. These results indicate that the representation of multiple objects in LOC is governed by a response normalization mechanism similar to that reported in visual areas of several nonhuman species. They also suggest a population coding scheme that preserves information about multiple objects under conditions of distributed attention, facilitating fast object and scene recognition during natural vision.

Integration of surface information in primary visual cortex

Ample evidence suggests that primary visual cortex is involved in the perception of form, and there is increasing evidence that it may also be important in the perception of surfaces. Perceptual qualities of surfaces, such as brightness, are based on extensive integration of information throughout the visual field. In primary visual cortex, we found that the responses of neurons to surfaces were also influenced by the intensity and organization of light in large portions of the visual field. Interactions with surrounding stimuli typically extended 10 to 20 degrees beyond a cells receptive field, the same spatial scale as perceptual interactions. Moreover, there were both facilitatory and inhibitory influences, just as there are additive and subtractive perceptual interactions. Surprisingly, influences from outside the receptive field obtained with surface stimuli did not reliably correlate with influences recorded with gratings. These properties suggest that the underlying neuronal interactions may serve as the fundamental building blocks of surface perception.

Eye-centered encoding of visual space in scene-selective regions

We used functional magnetic resonance imaging (fMRI) to investigate the reference frames used to encode visual information in scene-responsive cortical regions. At early levels of the cortical visual hierarchy, neurons possess spatially selective receptive fields (RFs) that are yoked to specific locations on the retina. In lieu of this eye-centered organization, we speculated that visual areas implicated in scene processing, such as the parahippocampal place area (PPA), the retrosplenial complex (RSC), and transverse occipital sulcus (TOS) might instead possess RFs defined in head-, body-, or world-centered reference frames. To test this, we scanned subjects while they viewed objects and scenes presented at four screen locations while they maintained fixation at one of three possible gaze positions. We then examined response profiles as a function of either fixation-referenced or screen-referenced position. Contrary to our prediction, the PPA and TOS exhibited position-response curves that moved with the fixation point rather than being anchored to the screen, a pattern indicative of eye-centered encoding. RSC, on the other hand, did not exhibit a position-response curve in either reference frame. By showing an important commonality between the PPA/TOS and other visually responsive regions, the results emphasize the critical involvement of these regions in the visual analysis of scenes.

The importance of modulatory input for V1 activity and perception

To conduct well-controlled studies of visual processing in the laboratory, deviations from natural visual situations must generally be employed. In some regards, the reduced visual paradigms typically used are adequate for providing an accurate description of visual representations. However, the use of fixation paradigms and stimuli isolated within a receptive field may underestimate the richness of visual processing in area V1. Experiments ranging from lightness encoding and perception to paradigms involving natural scenes and saccades used to examine the relationship between V1 activity and perception are reviewed in this chapter. Using more complex and natural visual stimulation, V1 responses have been detected that are significantly different from responses obtained in more reduced paradigms. A feature common to the findings of different experiments is that the scale of the activated neural population and circuitry appears to play a key role in the correlation between V1 activity and perception. More complex and natural visual stimulation brings into play extra-receptive field modulatory input not involved with stimulation localized to the receptive field. The results suggest that rather than subtly sculpting the response, modulatory input coming from intra- and/or intercortical sources is fundamental in establishing perceptual response patterns in natural visual situations.