Stephen A. Engel

Linear Systems Analysis of Functional Magnetic Resonance Imaging in Human V1

The linear transform model of functional magnetic resonance imaging (fMRI) hypothesizes that fMRI responses are proportional to local average neural activity averaged over a period of time. This work reports results from three empirical tests that support this hypothesis. First, fMRI responses in human primary visual cortex (V1) depend separably on stimulus timing and stimulus contrast. Second, responses to long-duration stimuli can be predicted from responses to shorter duration stimuli. Third, the noise in the fMRI data is independent of stimulus contrast and temporal period. Although these tests can not prove the correctness of the linear transform model, they might have been used to reject the model. Because the linear transform model is consistent with our data, we proceeded to estimate the temporal fMRI impulse–response function and the underlying (presumably neural) contrast–response function of human V1.

Remembering episodes: a selective role for the hippocampus during retrieval

Some memories are linked to a specific time and place, allowing one to re-experience the original event, whereas others are accompanied only by a feeling of familiarity. To uncover the distinct neural bases for these two types of memory, we measured brain activity during memory retrieval using event-related functional magnetic resonance imaging. We show that activity in the hippocampus increased only when retrieval was accompanied by conscious recollection of the learning episode. Hippocampal activity did not increase for items recognized based on familiarity or for unrecognized items. These results indicate that the hippocampus selectively supports the retrieval of episodic memories.

Colour tuning in human visual cortex measured with functional magnetic resonance imaging

The primate retina contains three classes of cones, the L, M and S cones, which respond preferentially to long-, middle- and short-wavelength visible light, respectively. Colour appearance results from neural processing of these cone signals within the retina and the brain. Perceptual experiments have identified three types of neural pathways that represent colour: a red–green pathway that signals differences between L- and M-cone responses; a blue–yellow pathway that signals differences between S-cone responses and a sum of L- and M-cone responses; and a luminance pathway that signals a sum of L- and M-cone responses. It might be expected that there are neurons in the primary visual cortex with response properties that resemble these three perceptual pathways, but attempts to find them have led to inconsistent results. We have therefore used functional magnetic resonance imaging (fMRI) to examine responses in the human brain to a large number of colours. In visual cortical areas V1 and V2, the strongest response is to red–green stimuli, and much of this activity is from neurons receiving opposing inputs from L and M cones. A strong response is also seen with blue–yellow stimuli, and this response declines rapidly as the temporal frequency of the stimulus is increased. These responses resemble psychophysical measurements, suggesting that colour signals relevant for perception are encoded in a large population of neurons in areas V1 and V2.

A Dissociation of Encoding and Retrieval Processes in the Human Hippocampus

The hippocampal formation performs two related but distinct memory functions: encoding of novel information and retrieval of episodes. Little evidence, however, resolves how these two processes are implemented within the same anatomical structure. Here we use high-resolution functional magnetic resonance imaging to show that distinct subregions of the hippocampus are differentially involved in encoding and retrieval. We found that regions early in the hippocampal circuit (dentate gyrus and CA fields 2 and 3) were selectively active during episodic memory formation, whereas a region later in the circuit (the subiculum) was active during the recollection of the learning episode. Different components of the hippocampal circuit likely contribute to different degrees to the two basic memory functions.

Learning Strengthens the Response of Primary Visual Cortex to Simple Patterns

Training can significantly improve performance on even the most basic visual tasks, such as detecting a faint patch of light or determining the orientation of a bar (for reviews, see ). The neural mechanisms of visual learning, however, remain controversial. One simple way to improve behavior is to increase the overall neural response to the trained stimulus by increasing the number or gain of responsive neurons. Learning of this type has been observed in other sensory modalities, where training increases the number of receptive fields that cover the trained stimulus. Here, we show that visual learning can selectively increase the overall response to trained stimuli in primary visual cortex (V1). We used functional magnetic resonance imaging (fMRI) to measure neural signals before and after one month of practice at detecting very low-contrast oriented patterns. Training increased V1 response for practiced orientations relative to control orientations by an average of 39%, and the magnitude of the change in V1 correlated moderately well with the magnitude of changes in detection performance. The elevation of V1 activity by training likely results from an increase in the number of neurons responding to the trained stimulus or an increase in response gain.

Perceptual learning in object recognition: object specificity and size invariance

A series of four experiments measured the transfer of perceptual learning in object recognition. Subjects viewed backward-masked, gray-scale images of common objects and practiced an object naming task for multiple days. In Experiment 1, recognition thresholds decreased on average by over 20% over 5 days of training but increased reliably following the transfer to a new set of objects. This suggests that the learning was specific to the practiced objects. Experiment 2 ruled out familiarity with strategies specific to the experimental context, such as stimulus discrimination, as the source of the improvement. Experiments 3 and 4 found that learning transferred across changes in image size. Learning could not be accounted for solely by an improvement in general perceptual abilities, nor by learning of the specific experimental context. Our results indicate that a large amount of learning took place in object-specific mechanisms that are insensitive to image size.

Adaptation of oriented and unoriented color-selective neurons in human visual areas.

Primary visual cortex contains at least two distinct populations of color-selective cells: neurons in one have circularly symmetric receptive fields and respond best to reddish and greenish light, while neurons in another have oriented receptive fields and a variety of color preferences. The relative prevalence and perceptual roles of the two kinds of neurons remain controversial, however. We used fMRI and a selective adaptation technique to measure responses attributable to these two populations. The technique revealed evidence of adaptation in both populations and indicated that they each produced strong signals in V1 and other human visual areas. The activity of both sets of neurons was also reflected in color appearance measurements made with the same stimuli. Thus, both oriented and unoriented color-selective cells in V1 are important components of the neural pathways that underlie perception of color.

Selective adaptation to color contrast in human primary visual cortex

How neural activity produces our experience of color is controversial, because key behavioral results remain at odds with existing physiological data. One important, unexplained property of perception is selective adaptation to color contrast. Prolonged viewing of colored patterns reduces the perceived intensity of similarly colored patterns but leaves other patterns relatively unaffected. We measured the neural basis of this effect using functional magnetic resonance imaging. Subjects viewed low-contrast test gratings that were either red–green (equal and opposite long- and middle-wavelength cone contrast, L-M) or light–dark (equal, same-sign, long- and middle-wavelength cone contrast, L+M). The two types of test gratings generated approximately equal amounts of neural activity in primary visual cortex (V1) before adaptation. After exposure to high-contrast L-M stimuli, the L-M test grating generated less activity in V1 than the L+M grating. Similarly, after adaptation to a high-contrast L+M grating, the L+M test grating generated less activity than the L-M test grating. Behavioral measures of adaptation using the same stimuli showed a similar pattern of results. Our data suggest that primary visual cortex contains large populations of color-selective neurons that can independently adjust their responsiveness after adaptation. The activity of these neural populations showed effects of adaptation that closely matched perceptual experience.

Unfolding the human hippocampus with high resolution structural and functional MRI

The hippocampus is a region of the brain that is crucial to memory function. Functional neuroimaging allows for the noninvasive investigation of the neurophysiology of human memory by observing changes in blood flow in the brain. We have developed a technique that employs high‐resolution functional magnetic resonance imaging (fMRI) in combination with cortical unfolding to provide activation maps of the hippocampal region that surpass in anatomic and functional detail other methods of in vivo human brain mapping of the medial temporal lobe. We explain the principles behind this method and illustrate its application to a novelty‐encoding paradigm. Anat Rec (New Anat) 265:111–120, 2001.

Neural Response to Perception of Volume in the Lateral Occipital Complex

Projection of a 3D scene onto the 2D retina necessarily entails a loss of information, yet perceivers experience a world populated with volumetric objects. Using simultaneous behavioral and neural (fMRI) measures, we identify neural bases of volume perception. Neural activity in the lateral occipital cortex increased with presentation of 3D volumes relative to presentation of 2D shapes. Neural activity also modulated with perceived volume, independent of image information. When behavioral responses indicated that observers saw ambiguous images as 3D volumes, neural response increased; when behavioral data revealed a 2D interpretation, neural response waned. Crucially, the physical stimulus was identical under both interpretations; only the percept of volume can account for the increased neural activity.