Ch 11 1st set of notes

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Ch 11 (1st set of notes)

• Psyc 317

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pick a card any card!

• Select one only of these six face cards.
• Keep it secret (dont tell anyone)
• I will now present five cards and the one you
  picked will be missing.
• Ready?

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Yours is missing isnt it?

• Whats the secret?
• You may have forgotten to check if the other
  cards from earlier are still here.
• Actually, none of the above cards were present on
  the previous slide.
• Moral we perceive not a scene but the gist of a
  scene.

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Time the neglected dimension in perception

• The assumption that retinal image processing is
  pointilistic, along with the usual procedures for
  studying snapshot vision (chin-rest, muscle
  relaxants, ultrabrief stimuli, etc.) has led
  until recently to a general neglect of the
  temporal parameters of visual perception.
• When time was brought into account, it was often
  a matter of adding up snapshots (as in the old
  reel films).
• But there is good reason to think of perception
  as the perception of events just as much as
  perception of objects.
• In fact, actions are particular kinds of events
• Also, event perception involves object perception
• Perception tells us about what is happening to
  or around me as also about who/what is doing
  what to whom/what
• Events point to future events (in ways that
  objects alone do not)
• Events are relations among objects and actions
• Events are series of stimuli that unfold over
  time

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Constant Change is Necessary for Perception

• In fact, the necessity of change for perception
  to occur has long been demonstrated.
• Starting in the 1950s, it was shown that a
  perfectly stabilized retinal image leads to
  uniform blur and effective blindness.
• For vision, there must be variations in
  illumination over time, as also spatial variation
  in the intensity of light (e.g. patterns of
  edges)
• The technique involves mounting tiny projectors
  onto contact lenses, so that every motion of the
  eye produces a corresponding (synchronized)
  motion of the retinal image.
• The result is that, no matter how the eye moves,
  the retinal image falls on the exact same part of
  the retina.
• Over a period of seconds, the entire visual field
  disappears from consciousness, with colors fading
  and contours falling away.
• Flickering a stabilized image will allow it once
  again to reappear.

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Microsaccades tiny eye movements that make still
objects visible

• A saccade is a jerking movement of the eye.
• Regular saccades happen when we flit our eyes
  from point A to point B.
• In addition, the eyes, even when intentionally
  fixated on a stationary object, also slowly
  drift
• We regularly if unconsciously revert our eyes to
  the chosen fixation point.
• Microsaccades are tiny jiggling movements that
  occur many times a second, even during successful
  fixation.
• The result is that the retinal image is
  constantly destabilized, and thus visible.
• The output of each retinal receptor, even during
  steady fixated vision, varies over time as the
  image shivers over the retina.
• Microsaccades allow perception of stationary
  objects
• They are a relatively late evolved mechanism not
  present in many animals, who can lose sight of
  objects not moving relative to them

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Percepts dont happen all at once they emerge
over time

• We live in the past.
• The specious present of experience is in fact a
  laggard.
• the owl of Minerva flies at dusk
• In the graphic at left, a 1 ms stimulus (a flash)
  over a tiny retinal area, causing neural activity
  to begin (in graph below).
• But this activity increases slowly, resulting in
  tiny delay of experience of the stimulus.
• More importantly, the neural activity goes on for
  some time, making the experience of the 1 ms
  flash last much longer (100-400 ms!) than the
  flash itself
• The falsely-long experience is called visible
  persistence and is thought to arise from the
  on-going neural activity.

Visible persistence can be thought of as a rival
to the iconic theory of the very short term
memory.
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Visual Sensory Memory (iconic memory)
A Q 6 8 T P W 1 2 Y 6 L

• Sperling (1960) Whole-report vs. Partial Report
• whole report technique
• subjects saw a display of letters for 50 msec
• subjects then asked to recall the items
• results subjects recalled 4 items
• partial report technique
• subjects saw display for 50 msec
• 50 msec later subject heard a tone indicating the
  row of the display to be recalled
• high pitched tone--top row
• medium pitched tone--middle row
• low pitched tone--bottom row
• results subjects recalled 3-4 items (regardless
  of the row)
• also, partial report advantage disappears if
  stimulus is masked
• Implication
• 9-12 items available in iconic memory - we see
  more than we remember!

Immediate Report 4 correct 33 of
array Immediate Cue of Row -3 correct implying
75 of array available
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Sperlings Results Partial Report and Iconic
Memory

• So why are only 4 items recalled in whole report
  technique?
• One idea the image of the items fades so
  rapidly that a person can only report 4 items
  before the rest of it fades from iconic memory

• To test this hypothesis, Sperling varied the
  delay between the offset of the letter matrix
  display and the onset of the tonal cue
• although 80 of the matrix could be reported when
  the cue was presented immediately, only about 50
  could be reported after ¼ of a second
• after about ½ second, recall had fallen to the
  rather meager levels seen in the whole-report
  condition
• the entire matrix was available initially, it
  faded rapidly, and was gone within about 500 ms

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Characteristics of Iconic Memory

• Large Capacity
• Up to 17 letters, up to 2 seconds (although
  duration is typically briefer, 500 milliseconds
  to 1 second)
• Capacity depends on stimulus conditions
• dark fields enhance, bright pre and post
  fields cut down duration
• Spontaneous Decay and Potential to be Erased
• Loss of info due to spontaneous decay (rather
  than interference)
• Decay begins at onset of target, not offset
  (DiLollo, 1980)
• Information can also be lost due to a following
  stimulus acting as a mask (Breitmeyer Ganz,
  1976 Turvey, 1973)
• Display followed by location cue ( _ or ?)
  circle wipes out target
• Precategorical Representation
• Initially thought to be a physical representation
  as selection of items on physical characteristics
  (ex. color) possible but not on semantic
  characteristics (digits vs. letters) (Sperling ,
  1960).
• This has been challenged there may be some
  semantic info in iconic memory

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Communicating over the vast expanse of the Brain

• It takes between 10-50 ms for visual signals to
  reach the thalamus (LGN).
• It takes another 20-50 ms for them to reach the
  primary visual cortex (V1) in the occipital lobe.
• It takes 50-100 ms for signals to get from V1 to
  the frontal centers involved in action and
  planning.
• E.g. a red traffic light goes on red light
  reaches the retina a signal is sent out
• 40 ms later it is in the LGN
• 20 ms later it is in V1
• 20 ms later it is in V4 (deals with color)
• 40 ms later it is in the frontal planning centers
  (traffic light is only just seen). Some time
  also required to decide to act.
• 10-15 ms is required to initiate a motor response
• At 100 kph, you have already traveled 6 meters,
  and your foot is just beginning to move toward
  the brake pedal.
• Unfortunately you may need to add the effects of
  alcohol and cell-phone distraction to get
  realistic timeframe.

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Flash Lag Effect

• The brain has mechanisms to compensate and
  (partly) correct for its slow rate of
  processing.
• It can sometimes anticipate where stimuli will be
  in advance.
• Its as if perception of smooth rotation is made
  possible by anticipation but no anticipation of
  changed location applies to the briefly flashed
  bars, so they are seen as behind, though they are
  actually perfectly horizontal.

Indeed, evidence exists that certain LGN cells in
cats that respond selectively to directional
motion require less input activity to do so when
they are receiving feedback from V1 cells. But
the higher centers must have had enough
information to develop an anticipatory
hypothesis (top-down)
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Temporal Integration

• Frame 1 and Frame 2 each contain twelve dots.
• They are presented (for 1 ms only) one after
  another in the same location with a variable
  inter-stimulus interval (ISI)
• If the ISI is short enough, the two patterns are
  integrated over time to produce the combined
  percept shown.
• Subjects are asked to identify the location where
  there is no spot.
• Results the longer the ISI, the worse subjects
  are at finding the missing dot.

With ISI from 0-50 ms, accuracy identifying the
missing dot is very high (subjects see one
pattern with 24 dots). At 100 ms, accuracy is
close to chance (4)
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Backward masking

• If two stimuli are briefly presented one after
  another in the same location, the second of the
  two is perceived more accurately than the first
  (backward masking)
• Bachman Allik (1976) used five shapes (at
  left), presented them for 10 ms, and asked
  subjects to identify them.
• Presented together, stimulus recognizability was
  low.
• Presented with a delay, stimulus recognizability
  varied as delay grew
• Accuracy identifying the second rose
• Accuracy identifying the first fell, but then
  picked up again for longer ISI.
• Apparently, competition arises for processing
  resources between the two.

? first shape ? second shape ?
? notice the J-shaped masking curve
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Spatial Analysis Requires Time

• Not only is stimulus change required for
  perception not only does the percept require
  time to arise but even spatial analysis of
  unchanging stimuli requires time.
• Why cant we detect the global inconsistencies
  immediately in the Escher engraving Belvedere?
• The reason seems to be that the fovea has only a
  very limited spatial range over which it can
  exercise is fine visual acuity.
• This allows it to verify local consistency of
  depth cues.
• To check for global inconsistencies, regions of
  the printed image must be compared.
• Only by comparing different regions can the
  correct spatial analysis of a scene be achieved
• Contrary to appearances, we do not see the whole
  at once
• This is related to the illusion of complete
  perception.
• It is also related to the idea that we perceive
  the gist of an entire scene.

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Temporal Resolution and the Visual System

• The sensitivity of the eye/brain to changes in
  illumination over time is highly variable, and
  depends on many factors.
• Retinal receptors can resolve flicker rates up to
  several hundred cycles per second (cps).
• The sensitivity of V1 neurons to change over time
  is much less
• A rapidly flickering light will be fused in
  consciousness into one light at various rates of
  flash, depending on the state of the system
• factors include currently level of dark or light
  adaptation intensity of the light distance of
  light from fovea wavelength composition retinal
  location stimulus size (see table 11.1 on p.
  337).
• The critical fusion frequency (CFF) is the
  measured frequency below which subjects are not
  able to distinguish a flickering light from a
  constant light.
• The CFF can fall anywhere between 10 and 60 cps.
• at 10 cps, the light is on for 50 ms durations
• At 50 cps, the light is on for about 20 ms
• at 60 cps, the light is on for 8.3 ms durations

The best CFF (highest resolution) is with
light-adapted eye and a high-intensity stimulus
in the periphery (any wavelength)
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Visual Pathways the Temporal Capacities of the
Visual System

• Recall that there are two visual pathways dorsal
  (action) and ventral (perceptual), and that the
  difference originates in the magnocellular vs.
  the parvocellular retinal ganglion cells.
• Recall too that neurons in magnocelllular system
  exhibit transient responses to stimuli, whereas
  the parvocellular exhibit sustained responses
• What might these different responses signify
  functionally?
• The magnocelllular system is largely responsible
  for time-keeping in vision.
• Distinguishing one 50 ms flash from two 50 ms
  flashes separated by a 50 ms interval is easy
  unless there is a irrelevant distracting 50 ms
  flash anywhere else in the visual field within
  about 250 ms of the test flash. Why?
• Leonard Singer (1997) varied the character of
  the distracting flash to tease apart the
  different roles of the magnocelllular and the
  parvocellular systems
• Low-contrast flashes activate the magnocelllular
  system
• Isoluminant flashes activate the parvocellular
  system
• High-contrast flashes activate both systems
• Low-contrast flashes disrupted accuracy for all
  kinds of test flashes whewreas accuracy for
  low-contrast test flashes were not disrupted by
  high-contrast or isoluminant distracters.
  What are the implications?

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Seeing where before seeing what

• seeing where something is happens quickly over
  time, many wheres perceived are motion
  perceived.
• Thus we can often tell who someone is when they
  are seen only as a walking silhouette (we
  recognize their individual gait, which is a
  moving or where stimulus).
• You can see a vehicle driving towards you from a
  long way away, well before you can tell its make,
  or even whether its a car or truck. Expert bird
  watchers rely on movement patterns rather than
  colour.
• Displays of randomly moving small bars can be
  manipulated (so that some of the bars (say in the
  central region), while still changing directions
  randomly, yet do so in a synchronized way.
• Even when the rate of synchonized changes of
  direction is 50 cps (every 20 ms) observers see
  the shape of the manipulated area, but cannot
  articulate how the the figure and the ground are
  different. (Unconscious?)
• These synchronized stimulus changes support the
  idea that neural synchrony is the basis of the
  perceptual now, consciousness, and communication
  among brain regions
• 20-60 cps oscillations of coincident neural
  firing may indicate that two brain regions are
  communicating about the same visual object

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Dichoptic Masking Results mask and target
presented to different eyes

• There is both forward and backward visible
  masking.
• The difference is which stimulus masks or
  interferes with the other.
• There is also simultaneous masking.
• Forward masking is weakest, occurring only if the
  first stimulus is 100ms or less from the second.
• Due to visible persistence, this is more or less
  identical experientially to simultaneous masking.
  (why?)
• There is also monoptic vs. dichoptic masking.
• The results of dichoptic masking experiments are
  shown at left.
• Dichoptic masking cannot be accounted for by
  early or retinal processing (why?)

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Motion Smear the result of visible
persistence
Wag a finger in front of your face. What do you
see? A one finger at each end of the motion, and
a smear in between.

• The two fingers perceived (when only one is
  waved), and the smear are the result of visible
  persistence.
• If you wave two fingers, it becomes even more
  difficult to count the apparent fingers, which
  become super-imposed on each other and difficult
  to disentangle perceptually.
• The smear interferes with object identification
  (shape, even colour) but it assists in projecting
  location (determining trajectory)
• In the dark, motion smear is even greater.
  (Why?)
• When motion is predictable, smear is suppressed.

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Perceptual Stability
Perception over time can be seen as a tension
between opposing forces perceptual stability and
perceptual plasticity

• Consider a constant tone that is interrupted, in
  one condition by silence, in others by an
  interval of white noise.
• The tone is perceived as absent during the
  silence, but it can retain a phenomenal
  presence during the white noise.
• Whether the tone is perceive in the interval of
  noise actually depends on what follows the
  interval.
• If the tone picks up after the interval, it is
  perceived as having been there all along. This is
  called the auditory continuity illusion (phonemic
  restoration)
• Compare the phenomenal presence of occluded
  objects in amodal completion

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Perceptual plasticitycontext affects visual
motion

• If subjects are presented with Frame 1, then
  Frame 2 (in row A, at right), they experience a
  black dot flashing on and off beside a grey
  square.
• If moving dots are added to an otherwise
  identical display (row B), subjects now perceive
  the target black dot as moving, and indeed
  disappearing behind the grey square.
• Motion is induced in the target dot by apparent
  motion in the surrounding context!

Id like anicetea said in winter or summer
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Perception of Time the now and the flow.

• The perceptual now (also called the subjective
  now, or the specious present) is a perpetual
  late-arriver.
• James called it the saddle-back of time with a
  certain length of its own, on which we sit
  perched, and from which we look in two directions
  into time (1890)
• But we also perceive time as passing, as flowing
  past, as not remaining present. This is complex,
  involving at least three aspects
• duration estimation
• order or sequence in time (perceiving
  simultaneity and successiveness)
• planning of ordered sequences of events
• We can achieve duration estimation either by
  biological clocks (relying on dedicated brain
  mechanisms) or by cognitive clocks (where time is
  read off cognitive processes, rather than a
  monitored directly

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Suprachiasmatic Nucleus
The SCN of the hypothalamus (located just above
the optic chiasm) is the basis of the biological
clocks that maintain circadian rhythms

• The sleep-wake cycle is the obvious biological
  regularity that has a basis in neural tissue with
  temporal properties.
• Human circadian sleep-cycle is slightly longer
  than 24 hrs. Without resetting of our clock each
  day (typically by exposure to sunlight), our days
  would be 25 hrs
• The pituitary hormone melatonin can also reset
  the circadian clock (Zeitgeber)
• There are in fact several biological clocks,
  regulating periodic changes in blood pressure,
  body temperature, pulse, as well as feeding and
  mating behavior in animals.
  Entrainment is resetting.
• Other cycles have longer periods estrus,
  menstruation, hibernation, etc.

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Short Term Timers
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