Topi Tanskanen

Coinciding early activation of the human primary visual cortex and anteromedial cuneus

Proper understanding of processes underlying visual perception requires information on the activation order of distinct brain areas. We measured dynamics of cortical signals with magnetoencephalography while human subjects viewed stimuli at four visual quadrants. The signals were analyzed with minimum current estimates at the individual and group level. Activation emerged 55–70 ms after stimulus onset both in the primary posterior visual areas and in the anteromedial part of the cuneus. Other cortical areas were active after this initial dual activation. Comparison of data between species suggests that the anteromedial cuneus either comprises a homologue of the monkey area V6 or is an area unique to humans. Our results show that visual stimuli activate two cortical areas right from the beginning of the cortical response. The anteromedial cuneus has the temporal position needed to interact with the primary visual cortex V1 and thereby to modify information transferred via V1 to extrastriate cortices.

The imprint of action: motor cortex involvement in visual perception of handwritten letters.

Humans are able to recognize handwritten texts accurately despite the extreme variability of scripts from one writer to another. This skill has been suggested to rely on the observers own knowledge about implicit motor rules involved in writing. To investigate the possible neural correlates of such an ability, we monitored with magnetoencephalography (MEG) the approximately 20-Hz oscillations originating from the motor cortex. The oscillations were more suppressed after visual presentation of handwritten than printed letters, indicating stronger excitation of the motor cortex to handwritten scripts. These results support the idea of embodied visual perception of handwritten scripts and the involvement of the motor cortex in the underlying action-perception link.

Neuromagnetic correlates of visual motion coherence

In order to characterize cortical responses to coherent motion we use magnetoencephalography (MEG) to measure human brain activity that is modulated by the degree of global coherence in a visual motion stimulus. Five subjects passively viewed two‐phase motion sequences of sparse random dot fields. In the first (incoherent) phase the dots moved in random directions; in the second (coherent) phase a variable percentage of dots moved uniformly in one direction while the others moved randomly. We show that: (i) visual‐motion‐evoked magnetic fields, measured with a whole‐scalp neuromagnetometer, reveal two transient events, within which we identify two significant peaks – the ‘ON‐M220’ peak approximately 220 ms after the onset of incoherent motion and the ‘TR‐M230’ peak, approximately 230 ms after the transition from incoherent to coherent motion; (ii) in lateral occipital channels, the TR‐M230 peak amplitude varies with the percentage of motion coherence; (iii) two main sources are active in response to the transition from incoherent to coherent motion, the human medial temporal area complex/V3 accessory area (hMT+/V3A) and the superior temporal sulcus (STS), and (iv) these distinct areas show a similar, significant dependence of response strength and latency on motion coherence.

From local to global: Cortical dynamics of contour integration

Processing of global contours requires integration of local visual information. To study the involvement of different cortical areas and the temporal characteristics of their activity in such integration, we recorded neuromagnetic responses to arrays of Gabor patches in which a proportion of the patches was oriented either tangentially or radially with respect to a global circular contour; arrays with random patch orientations served as control stimuli. The first responses at 60-80 ms around the calcarine sulcus were similar to all stimuli. Starting from 130 ms, responses to the tangential contours differed significantly from responses to control stimuli, and the difference reached its maximum at 275 ms. The most pronounced differences emerged around the parieto-occipital sulcus, precuneus, cuneus, and superior and middle occipital gyri. This pattern of cortical activity was similar irrespective of whether the local elements were radial or tangential to the circle; however, the differences were smaller for the radial contours and tended to start 20-30 ms later. Correspondingly, discrimination reaction times were shortest for the contours consisting of tangential elements. These results demonstrate two spatially and temporally distinct stages of visual cortical processing, the first one limited to local features and the second one integrating information at a more global level.

Face recognition and cortical responses: Effect of stimulus duration

To clarify the relationship between face perception and cortical activation, we manipulated the face recognition performance of 9 subjects by varying the duration (DUR) of stimuli while cortical neuromagnetic responses were recorded. A face image replaced a continuous pixel-noise mask for 17-200 ms, and the subject reported which of the pre-learned faces had been presented. Two cortical responses were clearly stronger to intact than phase-scrambled faces: the temporo-occipital response peaking at 140-200 ms (M170) and a more widely distributed response peaking at 200-500 ms (M300). For the shortest DURs (17-33 ms), face recognition was at chance level and the cortical responses negligible. For DURs of 50-83 ms, the proportion of recognized faces as well as the strength of cortical responses increased steeply. Recognition performance saturated at DURs of around 100 ms, whereas cortical responses continued to increase until the longest DUR of 200 ms. Amplitudes of both M170 and M300 were thus tightly correlated with recognition performance (r=0.98), but comparison of the increment rates as a function of DUR showed the recognition performance to have an even closer similarity to M170 than to M300. In single-trial analysis the variability of response strengths increased in a direct proportion to response amplitude, demonstrating the averaged responses to be composed of graded rather than of all-or-nothing-type single responses.

Dynamics of cortical activation in a hemianopic patient

Although residual vision in patients with cortical blindness is common, its brain mechanisms are poorly known. To study these mechanisms we measured neuromagnetic responses to visual stimuli in a patient with right posterior cerebral lesion and left visual field hemianopia. His vision had partially recovered with intensive training before our measurements. Compared with the processing in the healthy side, early occipital responses were attenuated for both passive viewing of checkerboard reversal patterns and a letter identification task. In both conditions there were prominent longer-latency responses at the right superior temporal cortex. We suggest that the activation in the superior temporal cortex can partially compensate for the failure to produce synchronized population responses at the early stages of visual cortical processing.