Kaoru Amano

Visual Field Maps, Population Receptive Field Sizes, and Visual Field Coverage in the Human MT+ Complex

Human neuroimaging experiments typically localize motion-selective cortex (MT+) by contrasting responses to stationary and moving stimuli. It has long been suspected that MT+, located on the lateral surface at the temporal-occipital (TO) boundary, contains several distinct visual field maps, although only one coarse map has been measured. Using a novel functional MRI model-based method we identified two maps-TO-1 and TO-2-and measured population receptive field (pRF) sizes within these maps. The angular representation of the first map, TO-1, has a lower vertical meridian on its posterior side at the boundary with the lateral-occipital cortex (i.e., the LO-2 portion). The angular representation continues through horizontal to the upper vertical meridian at the boundary with the second map, TO-2. The TO-2 angle map reverses from upper to lower visual field at increasingly anterior positions. The TO maps share a parallel eccentricity map in which center-to-periphery is represented in the ventral-to-dorsal direction; both maps have an expanded foveal representation. There is a progressive increase in the pRF size from V1/2/3 to LO-1/2 and TO-1/2, with the largest pRF sizes in TO-2. Further, within each map the pRF size increases as a function of eccentricity. The visual field coverage of both maps extends into the ipsilateral visual field, with larger sensitivity to peripheral ipsilateral stimuli in TO-2 than that in TO-1. The TO maps provide a functional segmentation of human motion-sensitive cortex that enables a more complete characterization of processing in human motion-selective cortex.

Mapping hV4 and ventral occipital cortex: The venous eclipse

While the fourth human visual field map (hV4) has been studied for two decades, there remain uncertainties about its spatial organization. In analyzing fMRI measurements designed to resolve these issues, we discovered a significant problem that afflicts measurements from ventral occipital cortex, and particularly measurements near hV4. In most hemispheres the fMRI hV4 data are contaminated by artifacts from the transverse sinus (TS). We created a model of the TS artifact and showed that the model predicts the locations of anomalous fMRI responses to simple large-field on-off stimuli. In many subjects, and particularly the left hemisphere, the TS artifact masks fMRI responses specifically in the region of cortex that distinguishes the two main hV4 models. By selecting subjects with a TS displaced from the lateral edge of hV4, we were able to see around the vein. In these subjects, the visual field coverage extends to the lower meridian, or nearly so, consistent with a model in which hV4 is located on the ventral surface and responds to signals throughout the full contralateral hemifield.

Neural Correlates of Induced Motion Perception in the Human Brain

A physically stationary stimulus surrounded by a moving stimulus appears to move in the opposite direction. There are similarities between the characteristics of this phenomenon of induced motion and surround suppression of directionally selective neurons in the brain. Here, functional magnetic resonance imaging was used to investigate the link between the subjective perception of induced motion and cortical activity. The visual stimuli consisted of a central drifting sinusoid surrounded by a moving random-dot pattern. The change in cortical activity in response to changes in speed and direction of the central stimulus was measured. The human cortical area hMT+ showed the greatest activation when the central stimulus moved at a fast speed in the direction opposite to that of the surround. More importantly, the activity in this area was the lowest when the central stimulus moved in the same direction as the surround and at a speed such that the central stimulus appeared to be stationary. The results indicate that the activity in hMT+ is related to perceived speed modulated by induced motion rather than to physical speed or a kinetic boundary. Early visual areas (V1, V2, V3, and V3A) showed a similar pattern; however, the relationship to perceived speed was not as clear as that in hMT+. These results suggest that hMT+ may be a neural correlate of induced motion perception and play an important role in contrasting motion signals in relation to their surrounding context and adaptively modulating our motion perception depending on the spatial context.

Spatial-frequency tuning in the pooling of one- and two-dimensional motion signals.

Cortical neurons that initially extract motion signals have small receptive-fields, and narrow orientation- and bandpass-spatial-frequency tuning. Accurate extraction of the veridical motion of objects typically requires the global pooling of the output of multiple local-motion units across orientation and space. We examined whether the narrow spatial-frequency tuning present at the local-motion level is preserved at the global-motion-pooling stage. Stimuli consisted of numerous drifting Gabor or plaid elements that were either signal (carrier drift-speed consistent with a given global-motion vector) or noise (drift speed consistent with a random, noise vector). The carrier spatial-frequencies of the signal and noise elements were independently varied. Regardless of the frequency of the signal elements, broad low-pass masking functions were obtained for both Gabor (one-dimensional) and Plaid (two-dimensional) conditions when measuring the threshold signal ratio for identification of the global-motion direction. For the Gabor stimuli, this pattern of results was also independent of the relative orientations of the signal and noise elements. These results indicate that in the global-motion pooling of one-dimensional and two-dimensional signals, local-motion signals of all spatial frequencies are pooled into a single system that exhibits broadband, low-pass tuning.

Human neural responses involved in spatial pooling of locally ambiguous motion signals

Early visual motion signals are local and one-dimensional (1-D). For specification of global two-dimensional (2-D) motion vectors, the visual system should appropriately integrate these signals across orientation and space. Previous neurophysiological studies have suggested that this integration process consists of two computational steps (estimation of local 2-D motion vectors, followed by their spatial pooling), both being identified in the area MT. Psychophysical findings, however, suggest that under certain stimulus conditions, the human visual system can also compute mathematically correct global motion vectors from direct pooling of spatially distributed 1-D motion signals. To study the neural mechanisms responsible for this novel 1-D motion pooling, we conducted human magnetoencephalography (MEG) and functional MRI experiments using a global motion stimulus comprising multiple moving Gabors (global-Gabor motion). In the first experiment, we measured MEG and blood oxygen level-dependent responses while changing motion coherence of global-Gabor motion. In the second experiment, we investigated cortical responses correlated with direction-selective adaptation to the global 2-D motion, not to local 1-D motions. We found that human MT complex (hMT+) responses show both coherence dependency and direction selectivity to global motion based on 1-D pooling. The results provide the first evidence that hMT+ is the locus of 1-D motion pooling, as well as that of conventional 2-D motion pooling.

Visual mismatch response evoked by a perceptually indistinguishable oddball.

Mismatch field (MMF) is an early magnetoencephalographic response evoked by deviant stimuli within a sequence of standard stimuli. Although auditory MMF is reported to be an automatic response, the automaticity of visual MMF has not been clearly demonstrated, partly because of the difficulty in designing an ignore condition. Our modified oddball paradigm had a masking stimulus inserted between briefly presented standard and deviant stimuli (vertical gratings with different spatial frequencies). Perceptual discrimination between masked standard and deviant stimuli was difficult, but the early magnetoencephalographic response for the deviant was significantly larger than that for the standard, when the former had a higher spatial frequency than the latter. Our findings strongly support the hypothesis that visual MMF is evoked automatically.

Conditional spatial-frequency selective pooling of one-dimensional motion signals into global two-dimensional motion

This study examined spatial-frequency effects on a motion-pooling process in which spatially distributed local one-dimensional motion signals are integrated into the perception of global two-dimensional motion. Motion pooling over two- to three-octave frequency differences was found to be nearly impossible when all Gabor elements had circular envelopes, but possible when the width of high-frequency elements was reduced, and the stimulus as a whole formed a closed contour configuration. These results are consistent with a view that motion pooling is controlled by form information, and that spatial-frequency difference is one, but not an absolute, form cue of segmentation.

Development of a generative model of magnetoencephalography noise that enables brain signal extraction from single-epoch data

We presented a method of rejecting sensor-specific and environmental noise during magnetoencephalography (MEG) measurement that enables the extraction of brain signals from single-epoch data. The method assumes a parametric generative model of MEG data. The model’s optimal parameters were determined from single-epoch data, and noise reduction was performed by the decomposition of data within the optimal model. We confirmed our method’s validity through multiple experiments. Moreover, we compared our method’s performance with that of several previous noise-reduction methods. Finally, we confirmed that the proposed method followed by spatial filtering reduced noise more efficiently.