Newsgroups: comp.ai.philosophy,sci.philosophy.meta,talk.philosophy.misc
From: David@longley.demon.co.uk (David Longley)
Path: cantaloupe.srv.cs.cmu.edu!rochester!udel!gatech!swrinde!pipex!peernews.demon.co.uk!longley.demon.co.uk!David
Subject: Re: Blindsight - What is it?
Distribution: inet
References: <3nl2ng$7ol$1@mhafa.production.compuserve.com>
Organization: Myorganisation
Reply-To: David@longley.demon.co.uk
X-Newsreader: Demon Internet Simple News v1.29
Lines: 322
X-Posting-Host: longley.demon.co.uk
Date: Wed, 26 Apr 1995 12:20:32 +0000
Message-ID: <798898832snz@longley.demon.co.uk>
Sender: usenet@demon.co.uk
Xref: glinda.oz.cs.cmu.edu comp.ai.philosophy:27259 sci.philosophy.meta:17539

In article <3nl2ng$7ol$1@mhafa.production.compuserve.com>
           100407.750@CompuServe.COM "Mike Jacobs" writes:

> Anyone out there tell me something about the phenomenon of 
> 'Blindsight'. Am told that it is changing our perception of 
> 'perception' but can't find any references. 
> Any brief descriptions and insights would be much appreciated
> 
> Mucho Gracias
> 
> Mike
I hope the following will prove helpful. See also a series I've put up 
on a conceptually  related  issue akin to  Stich's 'Fragmentation   of 
Reason' (Fragments of Behaviour  1 - 9,  alt.prisons,  sci.psychology,
sci.stat.edu etc).


What is blindsight?
Campion,-John; Latto,-Richard
U Liverpool, England
Behavioral-and-Brain-Sciences; 1985 Dec Vol 8(4) 755-757

Discusses  L.  Lutzemberger  et  al's  (1985)  article  (see  PA,  Vol 
74:16080)  on  blindsight  research, and faults them  for  failing  to 
provide  an  explicit definition of blindsight. Lutzemberger  et  al's 
reaction  time  (RT) paradigm is a useful way of  indicating  the  Ss' 
knowledge  of  a  stimulus because it avoids  both  response  strategy 
changes  and  the  differential sensitivity  problem.  In  questioning 
Lutzemberger  et  al's  conclusions drawn from  data  showing  spatial 
summation  across  the hemianopic boundary, it is  contended  that  it 
cannot  be claimed that this is an unconscious process and that it  is 
an   effect   of  nonstriate  cortex  without   evidence   of   lesion 
localization. 

On inferring blindsight from normal vision.
Lutzemberger,-L.; Marzi,-C.-A.; Tassinari,-G.
U degli Studi, Istituto di Neurochirurgia, Pisa, Italy
Behavioral-and-Brain-Sciences; 1985 Dec Vol 8(4) 754-755

Discusses J. Campion et al's (1983) article (see PA, Vol 74:16001)  on 
the  conceptual and methodological framework of  blindsight  research, 
and discusses research on a method that avoids 1 of the major  sources 
of  artifacts  that plague this type of research--shifts  in  response 
criteria.  The method uses light stimuli that can easily be shown  not 
to  be scattered in the intact field of hemianopic patients.  

Contributions to the study of "blindsight": I. Can stray light account 
for  saccadic  localization  in  patients  with  postgeniculate  field 
defects?
Zihl,-J.; Werth,-R.
Max-Planck-Inst   fur  Psychiatrie,   Neuropsychologische   Abteilung, 
Munich, West Germany
Neuropsychologia; 1984 Vol 22(1) 1-11

Investigated the role of stray light in 2 patients' ability to  locate 
a  target presented tachistoscopically in their  perimetrically  blind 
hemifield.  The  2  Ss,  who showed  homonymous  field  defects  after 
unilateral geniculostriate damage, were tested under different target-
background luminance conditions. Results demonstrate that the Ss could 
locate  the  target in their hemianopic field in the  absence  of  any 
stray  light  at  their field border, as  assessed  by  an  electronic 
photometer.  In this condition, the Ss never had any visual  sensation 
with  regard  to the illumination of the target. In  conditions  where 
stray light was available at the field border, the Ss could not locate 
the  target,  even  though  they always  reported  a  light  diffusely 
distributed  over  the  sphere of the perimeter when  the  target  was 
illuminated.  The  stray light emitted from the  target  at  different 
distances  from  the  field hindered the Ss in  locating  the  target. 
Accordingly,   the  "stray  light  hypothesis"  is  rejected   as   an 
explanation for the Ss' ability to locate a light target presented  in 
their perimetrically blind field. (16 ref) 

Contributions  to the study of "blindsight": II. The role of  specific 
practice  for  saccadic localization in patients  with  postgeniculate 
visual field defects.
Zihl,-J.; Werth,-R.
Max-Planck-Inst   fur  Psychiatrie,   Neuropsychologische   Abteilung, 
Munich, West Germany
Neuropsychologia; 1984 Vol 22(1) 13-22

Investigated  the effect of practice on saccadic localization  in  the 
perimetrically  blind field region by 3 patients  with  postgeniculate 
brain  damage (aged 33, 45, and 63 yrs). The analysis of the  saccadic 
responses of the 3 Ss, who had homonymous visual field defects, showed 
that  in  the  1st  sessions they did  not  choose  between  different 
locations  but,  rather,  preferred  a  consistent  magnitude  of  eye 
movements.  In  these sessions, no correspondence between  target  and 
mean eye position was found. After the Ss were forced to change  final 
eye  position in each trial, significant localization performance  was 
observed.  It  seems,  therefore, that the  choice  between  different 
locations  is a necessary, but not a sufficient, prerequisite for  the 
ability  to locate targets within the perimetrically blind  hemifield. 
Forcing the Ss to choose frequently between different locations within 
their  perimetrically  blind  hemifields may help them  to  use  their 
spared  localizations  ability  even though they  can  never  see  the 
target.   (13   ref)  

Is  blindsight an effect of scattered light, spared cortex, and  near-
threshold vision?
Campion,-John; Latto,-Richard; Smith,-Y.-M.
U Liverpool, England
Behavioral-and-Brain-Sciences; 1983 Sep Vol 6(3) 423-486

"Blindsight" describes visually guided behavior elicited by a stimulus 
falling  within  the scotoma (blind area) caused by a  lesion  of  the 
striate  cortex. It is concluded from a review of the literature  that 
blindsight studies have generally failed to control for  nonblindsight 
interpretations partly because of poor methodology and partly  because 
of  difficulties  in defining blindsight. Five experiments with  8  Ss 
investigated the extent to which Ss would exhibit performance  similar 
to blindsight when using scattered light as a cue. This was done  with 
3  hemianopic Ss (aged 24, 29, and 62 yrs) by manipulating the  amount 
of scattered and direct light coming from a stimulus and with 5 normal 
Ss by presenting targets within their blind spots. Results reveal good 
blindsight  performance when only scattered light was available  as  a 
cue. It is concluded that an adequate case for blindsight has not been 
made  and that it is probably impossible to demonstrate the  existence 
of blindsight on purely behavioral grounds. It is suggested that  what 
is  required  is  the establishment of  relationships  between  visual 
function and independent anatomical evidence. 22 peer commentaries and 
the present author's response to them are included. (3 p ref) 

Plasticity in human blindsight.
Bridgeman,-Bruce; Staggs,-David
U California, Psychology Board of Studies, Santa Cruz
Vision-Research; 1982 Vol 22(9) 1199-1203

Trained  an adult male with an unusually large cortical  scotoma  (the 
result of a traffic accident), leaving a 9Deg. hemifield of vision  in 
each  eye,  to point to an oscillating target  positioned  within  the 
scotoma.  The practice effect transferred to low-contrast targets  and 
to  stationary ones, although oscillating targets were always  located 
more accurately. Following practice, S reported improved confidence in 
visual  orientation.  Results are interpreted in the  context  of  the 
contrast  between  the functions of subcortical  and  cortical  visual 
systems  in normal humans. (19 ref) 

"Blindsight":   Improvement  of  visually  guided  eye  movements   by 
systematic practice in patients with cerebral blindness.
Zihl,-Josef
Max-Planck-Inst fur Psychiatrie, Munich, West Germany
Neuropsychologia; 1980 Vol 18(1) 71-77

Trained 3 patients (48-57 yrs old) with damage to the geniculo-striate 
visual  pathways  to  discriminate  different  positions  of   targets 
presented  briefly in the blind region of their visual field.  Results 
indicate  that  the ability to detect and  localize  stimuli  improved 
markedly after the systematic training of visually evoked saccadic eye 
movements.  It  is concluded that even in the absence of  the  striate 
cortex  detection and localization of stimuli remained  still  intact, 
and that this visual capacity is probably mediated by the extrastriate 
retionotectal  pathway. (French & German abstracts) (16 ref)  



Spatial summation in blindsight.
Stoerig,-Petra
Ludwig-Maximilians-U Munchen, Inst fur Medizinische Psychologie, Germany
Visual-Neuroscience; 1993 Nov-Dec Vol 10(6) 1141-1149

Blindsight  is  the ability of patients with lesions  in  the  striate 
cortex and its geniculate afferents to process visual information from 
their  visual-field  defects  although they insist  they  do  not  see 
stimuli presented in the defective area. Spatial summation curves were 
determined  in  circumscribed visual-field defects of  5  adults  with 
blindsight.  Under  light-adapted  conditions that  favor  the  color-
opponent  system,  increment  thresholds for  white  and  red  targets 
presented  on  a  white  background were measured  as  a  function  of 
stimulus  size (9-110 min arcs). In normal and  defective  hemifields, 
summation  was  linear  for red stimuli. Curves  measured  with  white 
stimuli  showed  some  nonlinearity  in the  normal  hemifield  and  a 
pronounced  eccentricity-dependent notch in the field defect.  Neurons 
mediating  sensitivity differ in summation properties  for  wavelength 
and  intensity information. (PsycLIT Database Copyright 1994  American 
Psychological Assn, all rights reserved)

Conscious visual perceptual awareness vs. non-conscious visual spatial 
localisation examined with normal subjects using possible analogues of 
blindsight and neglect.
Graves,-Roger-E.; Jones,-Brenda-S.
U Victoria, BC, Canada
Cognitive-Neuropsychology; 1992 Dec Vol 9(6) 487-508

Localization  of  unseen  stimuli was  produced  with  short-duration, 
pattern-masked, tachistoscopically presented targets in 3 studies with 
60 adult students. Conscious spatial attention (CSA) did not influence 
sensitivity  for the blindsight effect (i.e., impairment of  conscious 
visual  perception with preservation of visual  spatial  localization) 
but  did affect the criterion, according to a signal detection  theory 
analysis.  Unawareness  of stimuli in an unattended visual  field  was 
also  produced.  CSA  decreased awareness  sensitivity  and  increased 
criterion for unattended targets, leading to the neglect effect (i.e., 
preservation of conscious perceptual vision with impairment of spatial 
visual function). Greater left than right side neglect was found,  due 
to  lower  awareness sensitivity on the left. Blindsight  and  neglect 
effects  were  negatively  correlated, suggesting  that  both  effects 
reflect the function of the same visual localization system.  

Possible blindsight in infants lacking one cerebral hemisphere.
Braddick,-Oliver; Atkinson,-Janette; Hood,-Bruce; Harkness,-William; et-al
U Cambridge, Visual Development Unit, England
Nature; 1992 Dec Vol 360(6403) 461-463

Reports  the  case  of  2 infants in  whom  one  cerebral  hemisphere, 
including   both  striate  and  extra-striate  visual   cortex,   were 
surgically  removed in the 1st yr. Single conspicuous targets  in  the 
half-field  contralateral  to  the  lesion  could  elicit   fixations, 
implying detection and orienting by a subcortical system. In contrast, 
binocular  optokinetic nystagmus (OKN) showed a marked  asymmetry.  In 
normal  neonates,  fixation  shifts and OKN have both  been  taken  to 
reflect  subcortical control. Results are consistent with  subcortical 
control for fixation but not for OKN. 

Localization of unseen visual stimuli by humans with normal vision.
Meeres,-Sandra-L.; Graves,-Roger-E.
U Victoria, BC, Canada
Neuropsychologia; 1990 Vol 28(12) 1231-1237

30 university students were shown tachistoscopically presented stimuli 
(half contained an open circle at 1 of 6 locations, half were  blanks) 
at 4 durations. Short durations and a pattern mask reduced Ss' ability 
to  detect the stimuli. For trials on which the Ss reported  that  the 
circle  was absent, they nevertheless guessed the exact  locations  of 
the  targets  significantly  better than chance.  A  signal  detection 
theory analysis explained that Ss were more sensitive for the location 
decision than for the detection decision. These findings show that the 
blindsight phenomenon of localization of "unseen" stimuli,  previously 
reported  for cortically blind patients, also occurs for  humans  with 
normal vision. 

Blindsight  in  rodents:  The use of a "high-level"  distance  cue  in 
gerbils with lesions of primary visual cortex.
Carey,-David-P.; Goodale,-Melvyn-A.; Sprowl,-Erin-G.
U Western Ontario, London, Canada
Behavioural-Brain-Research; 1990 May Vol 38(3) 283-289

Two experiments examined the possibility that Mongolian gerbils ( N  = 
21)  with  bilateral lesions of the striate cortex could  use  retinal 
image  size as a distance cue on a jumping task. Systematically  wider 
or  narrower  probe  landing platforms  were  inserted  among  regular 
training  trials  with  a standard-sized landing  platform.  In  sham-
operated  and  destriate  Ss, narrower  platforms  tended  to  produce 
overjumps  of  the leading edge of the landing  platforms,  while  the 
wider probes tended to produce underjumps. The size of this effect did 
not differ between the 2 groups. Differences between this type of task 
and   traditional  size  discrimination  experiments  are   discussed. 

Wavelength sensitivity in blindsight.
Stoerig,-Petra; Cowey,-Alan
U Munich Inst of Medical Psychology, Fed Rep Germany
Nature; 1989 Dec Vol 342(6252) 916-918

Measured  spectral  sensitivity in the normal and blind  fields  of  3 
patients  in whom blindsight was previously demonstrated (P.  Stoerig, 
1987) and in 2 normal observers to determine whether the pathways from 
the  degenerated  hemiretina  could  process  wavelength  information. 
Spectral  sensitivity  differed between normal  and  cortically  blind 
hemifields only in terms of absolute sensitivity. The subpopulation of 
surviving  primate beta ganglion cells, the only known ganglion  cells 
to  provide  color-opponent signals, was  still  functional.  


Blindsight and insight in visuo-spatial neglect.
Marshall,-John-C.; Halligan,-Peter-W.
Neuropsychology Unit, Oxford, England
Nature; 1988 Dec Vol 336(6201) 766-767

Discusses  the dissociation between overt and covert perception  in  a 
case  of  visuo-spatial  neglect  in a 49-yr-old  female.  The  S  had 
sustained  right  cerebral  damage  and  failed  overtly  to   process 
information  in the hemispace contralateral to lesion and had  a  left 
homonymous hemianopia. The S was presented simultaneously with 2  line 
drawings of a house, in one of which the left side was on fire. The  S 
judged  that  the drawings were identical; yet when  asked  to  select 
which house she would prefer to live in, reliably chose the house that 
was   not   burning.  

Visual evoked potentials in the investigation of "blindsight."
Shefrin,-Sandra-L.; Goodin,-Douglas-S.; Aminoff,-Michael-J.
U California School of Medicine, San Francisco, US
Neurology; 1988 Jan Vol 38(1) 104-109

Recorded  long-latency  visual evoked potentials in  4  patients  with 
homonymous hemianopias, 1 of whom had clinical evidence of blindsight. 
Stimuli  consisted of different words that appeared randomly and at  a 
constant  angle  to either side of the center of a TV screen,  and  Ss 
responded  to  1  previously specified word  (the  target)  by  finger 
extension.  Target  stimuli in the intact hemifield elicited  a  well-
formed  P3  response  in all Ss, whereas stimuli in  the  blind  field 
produced no such response except in the S with blindsight. The earlier 
potentials  in  this  S  were larger with  stimulation  of  the  blind 
hemifield  than  the intact field. By contrast, a  P100  response  was 
present  only with stimulation of the intact field in this S.  Results 
indicate  that cognitive processing of visual stimuli can  occur  even 
when subjective awareness of such stimuli is absent, and suggest  that 
such  processing occurs independently of the geniculostriate  pathway. 

Spatial summation across the vertical meridian in hemianopics: A  test 
of blindsight.
Marzi,-C.-A.; Tassinari,-G.; Aglioti,-S.; Lutzemberger,-L.
U degli Studi, Padua, Italy
Neuropsychologia; 1986 Vol 24(6) 749-758

Examined blindsight (the above chance detection or discrimination of a 
visual stimuli by an S who denies having seen the stimulus) in 9  male 
and  11 female patients with a homonymous field loss due to  vascular, 
surgical, tumoral, or traumatic lesions. Ss were tested for  summation 
of pairs of flashes simultaneously presented to either the same or the 
opposite  hemifields across the vertical meridian. Unlike  normals,  a 
flash presented to the abnormal hemifield did not affect the  reaction 
time  (RT) to an identical flash presented to the intact  field.  Like 
normals,  Ss  showed  summation  within  1  hemifield.  A  single-case 
analysis,   however,  provided  unequivocal  evidence  of   interfield 
summation in 1 S, and evidence of summation in 1 testing session for 3 
other Ss. The small proportion of Ss showing blindsight may be related 
in  part to the relatively short stimulus duration and brief  stimulus 
exposure  durations  used. 



-- 
David Longley
