Polymodal motion processing in posterior parietal and premotor cortex: a human fMRI study strongly implies equivalencies between humans and monkeys.

In monkeys, posterior parietal and premotor cortex play an important integrative role in polymodal motion processing. In contrast, our understanding of the convergence of senses in humans is only at its beginning. To test for equivalencies between macaque and human polymodal motion processing, we used functional MRI in normals while presenting moving visual, tactile, or auditory stimuli. Increased neural activity evoked by all three stimulus modalities was found in the depth of the intraparietal sulcus (IPS), ventral premotor, and lateral inferior postcentral cortex. The observed activations strongly suggest that polymodal motion processing in humans and monkeys is supported by equivalent areas. The activations in the depth of IPS imply that this area constitutes the human equivalent of macaque area VIP.

The importance of seeing it coming: a functional magnetic resonance imaging study of motion-in-depth towards the human observer.

Despite their crucial biological relevance, the neural structures differentially activated by the detection of optic flow towards the observer remain to be elucidated. Here, we deploy functional magnetic resonance imaging with normal volunteers to locate the areas differentially activated when motion towards the observer is detected. Motion towards the observer, compared with motion away, showed significant activations (P<0.05, corrected for multiple comparisons), as assessed using statistical parametric mapping, in the lateral inferior occipital cortex bilaterally and in right lateral superior occipital cortex. The areas implicated do not extend into area V5 or subdivisions thereof.Our data suggest that the representations of motion towards the observer implicate perceptual and attentional mechanisms acting at early stages of visual processing in extrastriate cortex. From the standpoint of efficient biological engineering, it makes sense that such crucially important functions as object motion towards the observer would be computed in early visual processing areas. Further studies will be required to determine the extent to which the effects we observed in lateral occipital cortex reflect differential attention to different types of motion, as contrasted with the derivation of explicit representations of motion towards the observer.

On the benefits of not trying: brain activity and connectivity reflecting the interactions of explicit and implicit sequence learning.

Under certain circumstances, implicit, automatic learning may be attenuated by explicit memory processes. We explored the brain basis of this phenomenon in a functional magnetic resonance imaging (fMRI) study of motor sequence learning. Using a factorial design that crossed subjective intention to learn (explicit versus implicit) with sequence difficulty (a standard versus a more complex alternating sequence), we show that explicit attempts to learn the difficult sequence produce a failure of implicit learning and, in a follow-up behavioural experiment, that this failure represents a suppression of learning itself rather than of the expression of learning. This suppression is associated with sustained right frontal activation and attenuation of learning-related changes in the medial temporal lobe and the thalamus. Furthermore, this condition is characterized by a reversal of the fronto-thalamic connectivity observed with unimpaired implicit learning. The findings demonstrate a neural basis for a well-known behavioural effect: the deleterious impact of an explicit search upon implicit learning.

The influence of explicit instructions and stimulus material on lateral frontal responses to an encoding task.

In this functional magnetic resonance imaging study, we explored the effects of both stimulus material and encoding task demands on activation in lateral prefrontal cortex (PFC). Two factors were manipulated: material type and task instructions. Subjects encoded words or abstract figures (factor 1: stimulus type) and were required to make either a meaning-based or a form-based (letter or shape) decision about each stimulus (factor 2: task instructions). Abstract figures engendered significantly higher levels of right PFC activity than did words. This effect was seen for meaning-based and form-based processing tasks and was significantly greater for the former. We did not observe a differential response of left lateral PFC to verbal and pictorial material. A double dissociation, however, was found within left PFC. A ventrolateral region (within left inferior frontal gyrus) showed the highest levels of activity when words were processed according to their meaning whereas activity in a more dorsolateral region (within left middle frontal gyrus) was greatest when words were processed according to their form (constituent letters). We have therefore observed a main effect of material type in producing lateralized activation of frontal lobes, although the strength of this effect is sensitive to the nature of the task that subjects are asked to perform. Left-side lateral PFC activity is also sensitive to task instructions but this effect was specific to verbal material. The complex patterns of frontal effect counsel against any simple dichotomy of frontal function at the level of either material or task type.

The effect of sequence repeat time on auditory cortex stimulation during phonetic discrimination.

Acoustic noise generated by the MR scanner gradient system during fMRI studies of auditory function is a very significant potential confound. Despite these deleterious effects, fMRI of the auditory cortex has been successful and numerous practitioners have circumvented the problem of acoustic masking noise. In the context of auditory cortex fMRI, the sequence repeat time (TR) has the effect of altering the length of time during which the scanner is quiet. Indeed, the move to whole-brain fMRI makes the problem of acoustic noise more acute and points to the need to examine the role of TR and its influence on the BOLD signal. The aim of this study was to examine the effect of varying the TR time on activation of auditory cortex during presentation and performance of a phonetic discrimination task. The results presented here demonstrate that the influence of sequence repeat time is considerable. For a short repeat time it is likely that the noise from the scanner is a significant mask and hinders accurate task performance. At the other extreme, a repeat time of 9 s is also suboptimal, probably due to attentional effects and lack of concentration and not least because of the longer overall measurement times. The results of this study point to a complicated interplay between psychophysical factors as well as physical parameters; attention, acoustic noise, total duration of the experiment, consideration of the volume of acquisition, and overall difficulty of the task have to be assessed and balanced. For the paradigm used here, the results suggest an optimal TR of around 6 s for a 16-slice acquisition.

The neural correlates of person familiarity. A functional magnetic resonance imaging study with clinical implications.

Neural activity was measured in 10 healthy volunteers by functional MRI while they viewed familiar and unfamiliar faces and listened to familiar and unfamiliar voices. The familiar faces and voices were those of people personally known to the subjects; they were not people who are more widely famous in the media. Changes in neural activity associated with stimulus modality irrespective of familiarity were observed in modules previously demonstrated to be activated by faces (fusiform gyrus bilaterally) and voices (superior temporal gyrus bilaterally). Irrespective of stimulus modality, familiarity of faces and voices (relative to unfamiliar faces and voices) was associated with increased neural activity in the posterior cingulate cortex, including the retrosplenial cortex. Our results suggest that recognizing a person involves information flow from modality-specific modules in the temporal cortex to the retrosplenial cortex. The latter area has recently been implicated in episodic memory and emotional salience, and now seems to be a key area involved in assessing the familiarity of a person. We propose that disturbances in the information flow described may underlie neurological and psychiatric disorders of the recognition of familiar faces, voices and persons (prosopagnosia, phonagnosia and Capgras delusion, respectively).

Deriving numerosity and shape from identical visual displays.

We presented identical displays of three to five dots in a functional magnetic resonance imaging (fMRI) experiment with normal volunteers. Two distinct directed attention tasks were performed on these displays: In one condition, subjects assessed the numerosity of the display; in the other condition, they assessed the shape of the display. Decisions based on numerosity activated differentially striate and extrastriate visual processing areas as well as left inferior frontal cortex. Decisions based on shape derived from arrangement activated differentially temporoparietal cortex bilaterally, medial posterior cingulate cortex, and left dorsolateral prefrontal cortex. These divergent neural activations in response to identical stimuli suggest that attentional mechanisms are deployed in very different ways in rapid enumeration of visual objects and in linking spatially discrete elements to one form.