Sensory modalities Flashcards
Lecture 7A (19 cards)
Mammals have dichromatic colour vision
- most mammals lack the m cone
- trichromats - humans and some old world primates discriminate more colours than dichromats
- marine mammals don’t see colours - no L cones
colour vision deficiencies
- 5.8% males
- 0.5% females
- S gene is on chromosome 7, M and L gene on X chromosome
- Colour deficiency (mild), subjects have trichromatic colour vision in subjects
- colour blindness - severe colour deficiency, dichromatic colour vision, more men affected than women
Tetrachromatic colour vision
some species like birds and bees have richer colour vision
what is colour
- light stimulus - illuminating light
- visual stimulus - reflection, filtering
- colour is not a physical reality
- colour exists in our subjective experience as a result of neural activity in response to visual stimuli
spectral reflectance curves of visual stimuli
- light that is reflected from different object surfaces differs in its wavelength composition
- measured with spectrophotometer
colour coding and perception
neural correlates and mechanisms:
1. retina - small optical projection of scattered light of different wavelengths on retina, signals from 2 or more photoreceptor types enable subsequent comparison of wavelengths distributions in each point of the visual field
2. retina and beyond - colour coding interneurons in the P pathway, colour sensitive neurons in v4
3. behaviour - colour detection, colour discrimination, identification of objects, faces, visual scenes, colour categorisation
colour coding from retina to v1
- P and M ganglion cells project to different layers in LGN
- Segregation - in the processing of visual information, although P and M ganglion cells receive input from the same photoreceptors, they maintain their segregation by projecting to different layers in the LGN, the segregated projection from retina to LGN are also retinotopic
- P ganglion cells - projects to parvocellular layer in LGN, small RFs, slower conduction speed, higher acuity, poor response to transient stimuli, colour sensitive
- M ganglion cells - project to magnocellular layer in LGN, large RFs, higher conduction speed, sensitive to motion, lower acuity, no colour discrimination
- parallel pathways in the human/primate visual system
- only P cells not M cells, in the LGN show spectrally opponent responses to monochromatic light stimuli of different wavelength
- neurons of the M and P pathway project to different layers in V1
object in a scene recognised better than alone
- scene selective cortex processes scene information, forming expectations that are fed back to shape object representations in visual cortex
- explains perceptual effects such as filling in
- ventral stream - fMRI, multivariate representation of the objects’ category (animate/inanimate) in object-selective cortex strongly enhanced by the presence of scene cortex
- MEG - scene and object signals peaked at 320 ms after stimulus onset, 100ms later than object alone
hearing guided behaviours
- Detect and discriminate locations and movement of sounds sources
- Spatial orientation and navigation
- Echolocation
- Vocalisations for auditory communication, bird song and human language
airborne sound waves
1.Sound - pressure waves, movement of air particles set in motion by vibrating structure
2. Propagates in three dimensions, alternating compression and rarefaction of air, molecules move back and forth from regions of high pressure to low pressure
3. Sound can also propagate from the skull to the ear via vibrations
4. Measurements to characterise an auditory sound stimulus: sound frequency (reciprocal of wavelength and perceived as pitch/tone, Hz), amplitude ([perceived as loudness, relative strength of wave as transmitted vibration), phase (length of wave cycle), waveform (change of amplitude of auditory stimulus over time)
sound stimuli are transformed to vibrations in the air
- different to the retina, the ear does not spatially map the locations of sound and only sorts sounds by wavelength via tonotopic mapping in the inner ear
- airborne sounds waves and/or bone-conducted sound vibrations impinge on the tympanum and middle ear bones which then vibrate accordingly
- vibrations are amplified by the middle ear bones in order to transmit the stimulus to the oval window of the cochlea
tonotopic arrangement of hair cells
- mapping by sound frequency
- hair cells are tuned to narrow range of sound frequencies simply by their location along the basilar membrane
stereocilia
- help to stretch open the ion channels
- bending of the stereocilia (input zone)
- opening on nonselective ion channels that allow influx of K+ and Ca2+ ions
- depolarisation of the hair cell opens voltage gated Ca2+ channels in the base of the hair cell (output zone)
- neurotransmitter is released to excite afferent auditory interneurons from the cochlear nerve
auditory tuning curves and behavioural audiograms
- auditory interneurons
- electrophsiological recordings from six cells in the cat’s auditory nerve
- psychophysical experiments measuring hearing threshold
- comparisons between species show different sensitivity rangers
auditory pathway - from receptor to primary cortex
- most projections from the cochlea to the contralateral cortex via the cochlear nerve and nuclei
- each superior olivary nuclei of the brainstem receives inputs from both cochlear nuclei for first stage of binaural coding of the spatial location of a sound source
- firther tonotopic as well as spatial mapping in the inferoir colliculi are located in the medial geniculate nuclei of the thalamus and primary auditory cortex
retinotopic mapping using imaging techniques
12 visual areas in mouse cortex in which neurons are anatomically organised as retinotopic maps
how do sensory systems interact
multisensory interactions occur at various levels of brain organisation
mcgurk effect
lipreading used to avoid ambiguities
multimodal cues and decision making
- multiple cues from different modalities add information and reduce ambiguity when detecting or identifying an object
- guide behavioural sequences and decision making and direct attention
- cocktail party effect - ability to drive attention towards one stimulus filtered out from the noisy environment
