Sustained and transient channels

Transcription

1 Sustained and transient channels Chapter 5, pp

2 Some models of masking are based on two channels with different temporal properties Dual-channel models Evidence for two different channels in the visual system?

3 Outline Physiology M and P systems Spatial characteristics Temporal characteristics Projections to LGN and visual cortex Psychophysics Flicker detection vs. pattern identification Spatial response profiles Temporal response profiles Stimulus onset and offset Reaction times

4 Outline Physiology M and P systems Spatial characteristics Temporal characteristics Projections to LGN and visual cortex Psychophysics Flicker detection vs. pattern identification Spatial response profiles Temporal response profiles Stimulus onset and offset Reaction times

5 M and P retinal ganglion cells M Large dendritic trees (mainly parasol cells) Larger receptive fields High contrast sensitivity, early saturation P Smaller dendritic trees (mainly midget cells) Smaller receptive fields Lower contrast sensitivity, not saturating Midget and parasol ganglion cells (Dacey & Petersen, 1992, PNAS, 89, )

6 M and P retinal ganglion cells Croner & Kaplan (1995). Vision Research, 35, 7 24.

7 Projections to LGN M cells project to LGN layers 1 and 2 (magnocellular) P cells project to LGN layers 3 6 (parvocellular)

8 Temporal characteristics M: transient responses, short response latencies P: sustained responses, longer response latencies 1 impulse response 1 impulse response relative amplitude relative amplitude time, ms temporal stimulus profile time, ms temporal stimulus profile 1 stimulus contrast stimulus contrast time, ms response time, ms response 1 relative response relative response time, ms time, ms

9 Latencies Cleland et al. (1971). J Physiol., 217,

10 Projections Retina to primary visual cortex: M LGN 1 & 2 V1 4Cα P LGN 3 6 V1 4Cβ Segregation in the cortex: Dorsal / where pathway (M-based) Ventral / what pathway (P-based)

11 Schmolesky et al. (1998). J Neurophysiol., 79,

12 M P Temporal Fast Slow Transient Sustained Spatial Coarse scale Fine scale Psychophysical evidence for two channels?

13 Outline Physiology M and P systems Spatial characteristics Temporal characteristics Projections to LGN and visual cortex Psychophysics Flicker detection vs. pattern identification Spatial response profiles Temporal response profiles Stimulus onset and offset Reaction times

14 Flicker detection vs. pattern identification Absolute detection thresholds, flicker detection thresholds, and pattern recognition thresholds for gratings. Ratio of sensitivities for two types of presentation, counterphase and on off: Kulikowski & Tolhurst (1973). J Physiol., 232,

15 Flicker detection vs. pattern identification Flicker detection based on a mechanism twice as sensitive to counterphase than on off modulation (transient mechanism?) Pattern recognition based on a mechanism with equal sensitivities to counterphase and on off modulation (sustained mechanism?) Absolute detection thresholds determined by the most sensitive mechanism in a given stimulus condition

16 Spatial response profiles Contrast sensitivity to a target line flanked by two sub-thresholds lines, distance varied Flickered at 12Hz Two tasks: detect flicker / identify pattern Flicker detection: coarse-scale mechanism Pattern identification: fine-scale mechanism Breitmeyer & Julesz (1975). Vision Research, 15,

17 Temporal response profiles Contrast sensitivity to a target grating (5 6 ms) Sub-threshold grating presented at various stimulus onset asynchronies Low spatial frequencies: bi-phasic temporal impulse responses High spatial frequencies: mono-phasic temporal impulse responses Watson & Nachmias (1977). Vision Research, 17,

18 Abrupt vs. gradual onset and offset Contrast sensitivities as a function of spatial frequency Abrupt or gradual onset and offset Sensitivity is higher for abrupt presentation at low spatial frequencies Consistent with transiently responding mechanisms tuned to coarse spatial scales Breitmeyer & Julesz (1975). Vision Research, 15,

19 Summation at stimulus onset and offset Contrast sensitivities measured for a brief (4 ms) target grating Presented against a longer (800 ms) sub-threshold grating Low spatial frequency: summation at onset / offset High spatial frequency: summation during the whole 800 ms duration Low spatial frequency (2.0 cpd): High spatial frequency (7.6 cpd): Tolhurst (1975). Vision Research, 15,

20 Reaction times as a function of spatial frequency Reaction times longer as spatial frequency is increased (cf. the speeds of M and P systems) Similar increase in the latency of responses measured with MEG Williamson et al. (1978). Vision Research, 18,

21 Tolhurst (1975). Vision Research, 15, RT distributions Reaction times to near-threshold gratings Bimodal distributions for low spatial frequencies, suggesting transient responses to the onset and offset of the grating Unimodal distributions for high spatial frequencies, indicating sustained responses Low spatial frequency (0.2 cpd): Higher spatial frequency (3.5 cpd):

22 RT distributions flicker masking Bimodal RT distribution to a low spatial frequency (0.5 cpd) target (duration 500 ms) Unimodal RT distributions for the same target when it is masked with a flickering grating Flicker masks the transient mechanism, and the detection is based on the sustained mechanism Breitmeyer et al. (1981). Vision Research, 21,

23 Summary Both psychophysical and physiological evidence for two channels with complementary temporal and spatial properties Transient channel (M-system) coarse spatial scales high temporal sensitivity conveys information about the temporal changes in the stimulus Sustained channel (P-system) fine spatial scales tuned to lower temporal frequencies information for the analysis of stimulus form

24 Contrast sensitivity functions Robson (1966). JOSA, 56,