Movimento complesso: flusso ottica

Lettura Morrone, M.C., Burr, D.C., and Vaina, L. (1995). Two stages of visual processing for radial and circular motion. Nature 376, Morrone, M.C., Tosetti, M., Montanaro, D., Burr, D.C., Fiorentini, A., and Cioni, G. (2000). A cortical area that responds specifically to optic flow, revealed by function Magnetic Resonance Imaging. Nature Neuroscience 3, d'Avossa, G. et al. (2007). Spatiotopic selectivity of BOLD responses to visual motion in human area MT. Nat Neurosci 10,

Neural response in MSTd Duffy & Wurtz, 1991

Psychophysical evidence for detection of optic flow in humans?

Spatial summation

Signal-to-noise Predictions S α N sectors σ 2 α N sectors σ α √N sectors S/σ α √N sectors

Spatial summation in noise

Signal-to-noise Predictions S α N sectors σ 2 = k (constant) S α N sectors

Spatial summation of contrast

Two-stage motion analysis Σ Local motion detectors Contrast thresholded Complex motion integrator

Spatial summation of contrast in noise

Temporal summation of contrast sensitivity

Motion coherence sensitivity 10 secs noise signal

Summation constants

Segmentation has different temporal integration. Even a weak signal embedded in noise can be seen given a long enough view

Summation with large fields

Summation with large fields in noise

Spatial summation for flow is extensive

Spatial summation of contrast sensitivity of sinusoids Anderson and Burr, 1987

Interim Conclusions There exists psychophysical evidence for detectors tuned to complex optical flow. Evidence suggest that the detectors are “second-stage” after contrast thresholding. Temporal summation is extensive: ~ 3 sec. Spatial summation is extensive, ~ 70°.

A neural centre for optic flow?

How Functional Magnetic Resonance Imaging (fMRI) Works: Brain Increased brain activity produces a local increase in blood oxygenation Physics Oxygenated and De- oxygenated blood have different magnetic properties MR images are brighter where there is greater brain activity

Unfolding Superior Lateral Superior Posterior 1 cm

Expanding ring V1 V2 V3

Rotating wedge V1 V2 V3

MT V1

Demo in dos Dos Demo

FLOW (30 s) NOISE (30 s) 2 s Inverting Direction Rotation Radial Translation

MT Time (s) Response Inverting Rotation / Inverting Noise Continuous Rotation / Noise Gradually Changing Flow / Noise MC b a c d e f

Translation Flow SB

Spatiotopic coding of visual motion in human MT

Two motion signals separated by delay Motion signals integrate spatiotopically across saccades Melcher & Morrone Nat. Neurosc. 2003

Percent correct Summation of motion signals one fixation two different fixations Two motion signals separated by delay

Integration of motion at threshold across saccades is spatiotopic Where in the human brain does the spatiotopic integration occur?

Cortex David Hubel and Torsten Wiesel: Spatially localized receptive fields in primary visual cortex

RETINOTOPIC MAP Tootel et al, Science, 1982 Monkey anatomy

RETINOTOPIA: Neuroni vicini della retina poiettano a Neuroni vicini della corteccia ‘Retinotopic’ visual areas

Each visual brain area contains a map of the visual world and performs a different function.

S1 S2 S3 S4

FLOW - NOISE GDA Left VF – Right VF Left Hemisfere 15˚ 4.5˚ Human MT complex

Response is specific to controlateral stimuli

V1 No modulation of response by gaze

MT: Strong modulation of response by gaze

Gaze modulation of MT and V1 Central target Eccentric target D’Avossa et al. Nat. Neurosc. 2007

2X8deg 7 deg S1 S2 S3 S4 D’Avossa et al. Nat. Neurosc Block design: spatial selectivity with eccentric fixation F1F0F2

Spatial selectivity with central fixation

Spatial selectivity of BOLD responses in hMT is modulated by eye position D’Avossa et al. Nat. Neurosc. 2007

Spatial selectivity of hMT is invarient in external but not retinal coordinates. D’Avossa et al. Nat. Neurosc. 2007

Spatiotopicity of V1 and MT D’Avossa et al. Nat. Neurosc. 2007

Left MT

Indexes

Fig1

Fig 2

Fig 3

Fig 4

Conclusion A specific portion of the hMT complex shows spatiotopic tuning, suggestion that the transformation from retinal to external coordinates may occur within this structure. The spatiotopicity requires attention.

Experiment in dos Biological motion