Normal Auditory Perception

            Sound enters the ear through the pinna--the fleshy, outer portion of the ear.  It acts as a protective barrier against foreign materials/bodies and a funnel for sound waves.  The folds of the pinna help amplify high frequency waves.  The external auditory canal then conducts the waves to the tympanic membrane (eardrum).  The membrane vibrates with the displacement pressure exerted by the sound wave.  The wave then needs to be translated from an air medium to a liquid one in the cochlea (greater resistance).  The ossicles are three bones—the malleus, incus, and stapes—that are configured in such a way as to increase the pressure of the vibrations onto the oval window (a flexible membrane at the base of the cochlea).  The stapes is much smaller in area than the eardrum, which concentrates the pressure, thus increasing it.  This process is known as impedance matching. 

            The cochlea is divided into three main chambers/canals: vestibular canal, tympanic canal, and the cochlear duct.  The two canals are connected at a small window called the helicotrema at the apex.  The cochlea is completely filled with fluid.  The round window (flexible membrane) at the base of the tympanic canal allows for displacement of fluid.  In the cochlear duct, lies the organ of corti.  The Tectorial and Basilar membranes sandwich a lining of receptor hair cells that send impulses to the auditory nerve.  The inner hair cells are responsible for about 95% of output to the auditory nerve.  They are attached to both membranes (embedded), and their change in shape (as they are pulled and compressed) opens the ion channels that relay information.  The outer hair cells are responsible for about 5% of the output.  They are also attached to both membranes but are not fully embedded.  They are thought to encode weak sounds (close to threshold) and sharpen the responses of the inner hair cells.  From there, the auditory nerve sends the signals on to several different routes to the brain.

Adapted from Inner Ear Anatomy: http://www.dizziness-and-balance.com/images/master-ear.jpg

Characteristics of Auditory Hallucinations

            The experience of auditory hallucinations varies greatly depending upon the individual who has them. Julie A. Holland, M.D. and Kevin C. Riley, PhD conducted a study that attempted to characterize types of auditory hallucinations in schizophrenic patients. The following data was obtained by interviewing 86 psychiatric inpatients at a major metropolitan teaching hospital in Philadelphia, PA. Patients from the hospital who had recently experienced auditory hallucinations were asked to participate as subjects. Priority was given to actively hallucinating patients. Patients were also excluded if their primary language was not English. Subjects underwent a detailed structured interview to evaluate the form, content, frequency extent, and intensity of the hallucinatory experiences.

The Experiment:

            Eighty-seven subjects completed the structured interview, 39 women and 48 men. Ages ranged from 17 to 75, with a mean of 36 years. The average education of the subjects was 11 years, ranging from 5 to 18. Sixty percent of subjects were either unemployed or unskilled laborers. All subjects were medicated, most commonly with anti-psychotics (85%) and/or benzodiazepines (41%). Twenty percent of the subjects were on anti-depressant medication as well. Urine drug screens were performed on roughly half of the subjects as part of the admissions work up, with 7 urines showing up positive for cocaine, 1 for opiates, and 0 for methamphetamine. Neurological studies were performed on 21 subjects; of those, two had asymetrical ventricals and one had frontal lobe atrophy.

            About one half of the interviewed group carried a diagnosis of schizophrenia, while the rest of the subjects carried a diagnosis of mood disorders or toxic/organic states, such as substance induced psychotic disorders or psychosis secondary to general medical condition. The subjects were broken down into two categories in order to test so hypotheses about the content of their hallucinations. There was no difference between groups in any of the demographic data except age, The toxic/organic group 46 years, and was significantly older than the schizophrenic and mood disorder patients (mean ages 36 and 33 years, respectively).

Meaningfulness of Content:

            Ninety-nine percent of our sample heard some speech at one time. Twenty percent indicated that at times they heard non-vocal stimuli such as noises, ringing, buzzing, music, or other. Eighty-three percent of the sample could understand what was being said when hallucinations were verbal While 7% of the sample hears commands, 48% overheard conversations regarding themselves, and 61% noted being called names. Only 19% of the samples endorsed hearing a narrative of their actions. Accusations (34%), threats (34%), and niceties (31%) were also reported.

            Of the patients who reported conversations, 36 percent described the conversations as about them, while 23% said that they were not about them, and 29% described their conversation as including them, in that they spoke their voices and their voices spoke back to them. Fifty-six percent said they recognized the voices: 39 % friends, 29% family, and 14% religious (angels, Jesus Christ, or God) and 13 percent occult. No significant differences across the diagnostic groups were observed in terms of the content of the hallucinations.

Hallucinatory Experience

Gender of Speaker       

Fifty-two percent of subjects could not ascribe a gender to the voice, and considered it to be a “mix,” while 24% considered the voice to be male and 5% female. Five percent could not choose any category. There was a non-statistical trend towards male subjects hearing men’s voices.

Number of Voices

The number of voices averaged 4.3, with no difference seen between the three groups. When asked if the multiple voices spoke at the same time, 51% said different things were said once, while 28% said the voices took turn speaking, and 11% said that multiple voices spoke in unison.

Origin of Hallucinations

Thirty-eight percent of subjects surveyed described the sound as stereophonic, while six percent said it came from the right side, and four percent said the left. Ten percent ascribed the origin of the sound as coming from behind them, and nine percent said from the front. Thirty-three percent of interviewed subjects chose “other.”

When asked to choose between inside and outside of the head, the split was nearly equal, with 51% endorsing inside, 40% choosing outside, and 9% describing a combination of the two. Seventy-nine percent reported the sound as originating in both ears, with 4% right and 9% left ears. Interestingly, when subjects were asked if they heard the same thing in each ear, 74% endorsed, while 26% denied this assumption.

Case Study

1. Most prominent when patient is exposed to traffic noise
2. Second most commonly heard in stressful social situations
3. least experienced when patient is exposed to "white noise" (static or constant, monotone sounds)

(Perona-Garcelan and Cuevas-Yust, 1997)

www.psychologyinspain.com/content/full/1998/1bis.htm

Patients' Perspectives

1.     Perceval was a schizophrenic patient who suffered from auditory hallucinations.  He claimed that the voices belonged to relatives and friends.  Most of the voices "were heard in my head."  However, many times he would hear them coming from the "air, or in different parts of the room."  Unlike many sufferers, Perceval experienced pleasant hallucinations; he heard singing and joyful conversation (Sometimes even "voices of contrition").  He claimed to hear over 14 distinct voices. ("Percevals Narrative" in Bateson, 1974)
2.     One patient (anonymous) spoke of his/her hallucinations that resulted in feelings of paranoia.  He/she thought that the voices were coming through the walls, washing machine, and the dryer.  The patient believed this to be a conspiracy contrived by government agencies to spy on him/her. (Anonymous, 1996)