sampling Flashcards
how do our sensory systems get information from the environment?
through sampling
we do not process all the sensory information around us
rather, the visual scene is sampled to extract relevant information
what limits are there to sensory and perceptual inputs
the optics of the eye blur the image formed on the retina due tot he diffraction that occurs as a result of the density changes of photoreceptors across the retina and varying RFs across our field of view
spectral variation is only sampled by three classes of cone receptor
sensitivity of the skin to two point discrimination differs oer the body
what errors are there in how the eye collect light
Formation of an upside down image on the retina
Errors in image formation (blurring) can alter the image formed on the retina with detrimental consequences for object recognition → systematic errors (Young et al., 2011).
what errors are there in how the eye senses light
spatial frequency variation is too small to be picked up by the cone sampling scale of the photoreceptor mosaic
the way we see an object depends on how we smaple it through our photoreceptors
motion may allow us to pick up fine detail - spatiotemporal pattern of activation
how is the fovea an anatomical specialisation
area of acute vision
specialisations such as thinner retinal surface (foveal pit), dense cones, no rods, displacement of other visual cells allow light to enter the fovea
is the retina uniform
no,
at the fovea cones are small and densely packed
as the degrees of visual angle increase, cones are larger and rod cells are interspersed
at the greatest retinal eccentricity, there are few spares large cones
what limits spatial resolution
spatial resolution is limited by the density of receptors and by convergence at later processing stages
why is rod spatial vision poor
saturation during daylight
convergence of many rod cells onto a single bipolar cell
what are the spatial limitations of the periphery
using a stimulus that at a large spatial scale has more distinct high contrast vertical bars but at low spatial scale has more distinct continuous squares as we cannot resolve the high spatial frequency anymore
this alteration of the percept occurs earlier in the periphery than when presented in the fovea
how do we compensate for the fall-off with eccentricity
The anstis chart
When the centre of the display is fixated each letter is at 10x its threshold legibility
A reciprocal relationship to a first approximation
what do retinopy and cortical magnification refer to
Visual projections are retinotopic such that topological relationships are maintained (Tootell et al., 1988)
A transformation occurs such that more central regions are given an increasing proportion of the representation as the signal proceeds.
Due to transformation in cone→ ganglion cell projections and ganglion cell → strait cortex projections
how do magnification factors change with eccentricity
Me= Mf/(1+E/Es)
where E = eccentricity
Mf = a constant showing the value at the fovea (ca. 1 cm/deg)
Es= the eccentricity where magnification has fallen to half its foveal value (ca. 0.3 and 0.9 deg)
The distance on the cortical surface that corresponds to one degree of visual angle (Wilson et al. ,1990)
Reciprocal relationship between eccentricity and the magnification factor at each eccentricity.
Scaling stimulus size by this factor can compensate for changes in neural processing (m-scaling)
what is crowding
aka. lateral masking: neighbouring objects have a devastating effect on object recognition
different tasks require different scaling to equate performance
fixation at a central focus point results in muddling of letters
what is meant by critical spacing (Bouma, 1978)
individuals were asked to identify single alphabetic letters at various locations in the visual periphery
for the a, the letter was presented in isolation
for the xax the letter was presented with flanking x letters on either side
even though the flanking letters were the same on every occaision their presence profoundly affected the ability to identify the target letter
critical spacing is proportional to eccentricity
△E=bxE where b is Bouma’s constant
spacing, not size is the key parameter - position in visual field so cannot just scale the stimulus
is peripheral vision bad
Many visual functions show a decline in performance for stimuli that are placed more eccentrically.
For some functions the decline can be compensated for by a simple scale factor
However different tasks require different scale factors
Moreover crowding depends not on an object size but on object spacing so simple scaling is not sufficient.
Peripheral vision is specialised for monitoring for change in the visual environment to show improved peripheral performance e.g flicker and movement.
Suggests peripheral vision quantitatively and qualitatively different from foveal vision because it is anatomically and functionally specialised for different tasks from foveal vision.
how do we overcome the nonuniformity of the retian
Detail is lost when the fovea is not focused on it, but this can be resolved by moving our eyes.
what is meant by passive and active vision
passive vision argues that the subjective impression of an immediate full-detail, pictorial view of the world should be approached with scepticism
this view downplays the inhomogeneity of the retina and emphasises the inner screen idea where the purpose of vision is to form a pictorial internal mental representation
active vision argues that the subjective impression is an illusion created through our ability to actively direct our eyes effortlessly to any desired location
what is the oculomotor system (land et al., 1999)
the system that moves our eyes
active vision relies on continual sampling through gaze redirection
eye movements are often the principal means of supporting active vision
Visual fixation precedes motor manipulation such that eye movements are used to seek out information in advance of each motor act rather than being slaves to the motor program.
what is the target-elicited saccade
Target elicited saccades occur when a target makes a sudden appearance outside the fovea.
Saccade latency (SL): time between the appearance of the target and the time that the eye starts to move. The RT of the eye response. Represents the cumulative time taken by the brain processes that enable orienteering.
When targets are suprathreshold, the properties of the target have very little effect on saccade latency.
However, saccade latencies are increased by additional cognitive loads.
what is the gap-effect and fixation disengagement
Gap sequence: the fixation stimulus disappears before the appearance of the peripheral target.
Overlap sequence: the fixation stimulus doesn’t disappear until after the appearance of the peripheral target.
SL decreases with longer gaps and increases with longer overlap.
Knowing in advance where the target will appear does not decrease the effect
This suggests that the offset of the fixation initiates some general preparatory process with the spatial program imposed later.
The preparatory process comprises two components
A general alerting component found with any warning signal (which also facilitates manulate RT)
Fixation disengagement
effect specific to ocular orienting
what types of eye movements are there
Driven by different stimuli, behaviours and neural controls Visual stabilisation Vestibulo-ocular reflex (VOR) Optokinetic reflex (OKR) Target selection Saccadic system Pursuit system Vergence system Fixation eye movements Drift Tremor Microsccades
what are task-dependent patterns of fixation (Yarbus, 1967)
Track eye movements as individuals observe a painting
Presented participants with a series of questions
People naturally concentrate on the faces and areas of spatially detail
Stereotyped eye movement responses depending on the information we want to see
We are actively sampling the image for goal-relevant information
what are saccadic eye movements like in reading
During reading we make very stereotyped movements
Typical saccades: 6-9 letter spaces
Typical fixations 200-250 ms
Regressions: 10-15%
As text difficulty increases, fixation durations increase, saccade lengths decrease and regression frequency increases.
Fixation duration can range from <100ms to >500ms
Saccade length can range from one letter space to over 15
Variations depend on processing demands and have allowed inferences about the cognitive processes involved in reading.
what stereotyped fixations are seen in face perception (Adolphs et al., 2005)
Saccades and fixations made by a normal control subject when judging emotion in sample expressions.
Normal observers show typical patterns of fixation, mainly around the eyes.
Patient SM with bilateral amygdala damage fails to fixate on the eyes. Also impaired on emotion recognition, particularly fear.
This suggests that normal patterns of eye movements are critically linked to extracting the relevant information for the perception of facial expressions of emotion
what are smooth pursuit eye movements Boman & Hotson, 1992
tracking a stimulus moving smoothly in a horizontal left-right-left-right regular sequence
eye position shows periods of smooth movement interrupted by occaisional small catch up saccades
the catch up saccades show up as brief peaks in eye velocity
changes in direction are anticipated
what drives fixation (Putnam et al., 2005)
We move our eyes so that our target falls on the fovea
The area of highest cone density is not always used for fixation
Over 100 fixation trails the repeatability of fixation locations was very accurate with an error of 17 micrometres on the retinal surface.
(does this suggest tuning/learning?)
what is troxler fading
When we keep our eyes still and fixated on the central point, surrounded by lilac ring the ring fades.
Can’t inhibit eye movements enough to prevent that stimulus from moving over different photoreceptors.
Can inhibit fixational eye movements enough to prevent the outer ring to move across a large enough area to move into different receptive field regions in the periphery as out spatial acuity is poor
what is the role of microsaccades?
critical in maintaining target visibility during fixation
may counteract visual fading
Participants were presented with a stimulus, and had to press a button when the stimulus has faded and another button when it reappears
micro-saccades have been linked to attentional orienting (Hafed & Clark, 2002)
Have a role in enhancing fine spatial detail (Ricci et al., 2007)
summary
sensory systems are selective in the information sensed
functional specialism of the sensory surface means that different regions serve different purposes
eye movements are importnant because of the difference between foveal and peripheral vision
peripheral vision is anatomically and functionally specialised for different tasks from foveal vision
the oculomotor system produced a range of specialised movements o the eyes
Stabilisation eye movements are reflexive
Saccadic eye movements are important for target selection - they are closely related to attentional processes
Smooth pursuit eye movements arise when tracking a moving target
Fixational eye movements counteract visual fading - without them we wouldn’t see at all.
