HLAA Hearing Loss Magazine July/August 2010 - 18

Severe Hearing Loss
continued from page 17

	 Therefore,	a	listener	does	not	 experience	the	sensation	of	a	smooth	 loudness	transition	from	soft	to	loud	 sounds.	The	resulting	subjective	experience	is	that	of	a	rapid	growth	in	the	 loudness	of	input	sounds,	from	barely	 hearing	a	sound	to	it	quickly	being	 too	loud.	This	phenomenon,	known	 as	“recruitment,”	is	one	of	the	key	 challenges	facing	an	audiologist	when	 fitting	hearing	aids	to	someone	with		 a	severe	hearing	loss.

Cochlear Dead Spots Affect Success with a Hearing Aid
Complicating	the	situation	is	the	 fact	that	the	presumed	thresholds	of	 someone	with	a	severe	hearing	loss	 may	not	be	accurate	because	of	the	 presence	of	cochlear	dead	regions	 (which	implies	damage	to	the	inner		 as	well	as	the	outer	hair	cells).	 	 People	with	cochlear	dead	regions	 often	report	hearing	clicks,	hisses,	or	 static	during	an	audiometric	examination,	rather	than	a	tonal	sensation,	 suggesting	that	the	specific	hair	cells	 usually	“tuned”	to	this	frequency	are	 damaged	or	dead.	 	 Research	has	been	conducted	on	 this	phenomenon	and	a	simple	test	 has	been	devised	to	test	for	its	presence	(the	Threshold	Equalizing	Noise	 test).	The	likelihood	of	dead	spots	 increases	as	a	hearing	loss	exceeds		 70	dB,	but	even	then	reports	suggest	 that	the	incidence	rate	rarely	exceeds	 60	percent.	 	 It	seems	that	it	is	the	people	with	 a	severe,	high-frequency	hearing	loss	 who	are	the	ones	most	likely	to	have	 this	problem.	The	presence	of	cochlear	 dead	spots	will	likely	affect	how	well	 someone	does	with	a	hearing	aid.	For	 example,	if	the	evidence	suggests	that	 dead	spots	exist	in	a	high-frequency	 range,	one	should	provide	amplified	sound	in	this	area	cautiously	 and	only	with	suitable	comparisons.	 This	can	be	done,	for	example,	by	 programming	one	of	the	hearing	 aid’s	memories	to	cut	off	the	higher	 frequencies	(where	the	dead	spots	are	 found),	while	another	memory	can	be	 programmed	to	include	these	higher	
18 Hearing Loss Magazine

frequencies.	This	way	the	user	can	 directly	compare	the	two	conditions.	 	 While	more	difficult,	people	with	 severe	hearing	loss	have	been	successfully	fitted	with	hearing	aids	for	years.	 The	goal	is	to	“package”	the	amplified	 sound	between	the	person’s	severely	 impaired	hearing	thresholds	and		 the	loudness	tolerance	levels	(the	 “dynamic	range”).	We	want	someone	 to	be	able	to	hear	soft	input	sounds	as	 well	as	possible,	while	simultaneously	 not	making	sounds	too	loud.	The		 narrower	the	dynamic	range,	the		 more	difficult	the	hearing	aid	fitting.	 	 One	way	to	package	a	wide	range	 of	input	sounds	into	a	restricted	 dynamic	range	is	to	use	a	hearing	aid	 feature	called	wide	dynamic	range	 compression	(WDRC),	included	 in	most	hearing	aids.	This	feature	 increases	the	degree	of	amplification	(gain)	of	low	input	sounds—to	 ensure	that	they	can	be	heard—and	 decrease	the	gain	of	higher	level	input	 sounds—to	be	sure	that	sounds	do	 not	become	unpleasantly	loud.	

be	activated	for	incoming	speech		 signals	(the	time	constants).	 	 These	are	not	yet	settled	issues	as	 far	as	I	can	see.	When	one	“compresses”	a	relatively	large	range	of	speech	 inputs	into	a	small	area,	the	signal	 will	consequently	be	distorted	(albeit	 intentionally)	to	a	certain	extent.	 	 Often	this	is	a	desirable	outcome;	 some	people	with	severe	hearing	loss	 can	use	the	acoustic	information	provided	in	this	narrow	range	to	increase	 their	speech	perception	capabilities.	 	 Other	people	with	severe	hearing	 loss	will	not	be	able	to	benefit	from	 highly-compressed	speech,	possibly	 because	in	addition	to	the	elevated	 hearing	thresholds,	the	damage	to	 their	hair	cells	also	produces	various	 kinds	of	psychoacoustic	distortions. 	 This	latter	group	of	people	apparently	prefers	a	linear	setting,	where	 a	similar	amount	of	gain	is	provided	 for	all	input	levels	until	the	output	 limits	of	the	hearing	aid	are	reached.	 It	is	only	at	this	point	that	the	hearing	aid’s	gain	is	reduced.	While	softer	 sounds	may	not	be	audible	in	this	 approach,	that	portion	of	the	input	 sounds	that	can	be	perceived	is	relatively	undistorted.	 	 There	are	studies	that	support	 both	approaches,	as	we	would	expect	 because	of	the	usual	range	of	individual	differences.	The	solution	to	this	 dilemma,	similar	to	the	one	made	 above	in	regard	to	frequency	range,	 is	to	devote	one	hearing	aid	program	 to	WDRC,	and	the	other	to	a	linear	 mode	of	amplification	(called	compression	limiting).	That	way	a	person	 can	compare	both	approaches	and		 use	the	one	he	or	she	prefers.	

Directional Microphones
There	are	several	other	potentially	 helpful	features	that	can	be	included	 in	a	hearing	aid	for	a	person	with	a	 severe	hearing	loss.	A	few	years	ago,		 it	was	thought	that	directional	microphones	would	not	be	helpful	to	such	 people.	Some	recent	research	,	however,	suggests	that	those	with	a	severe	 hearing	loss	can	derive	benefit	from	 directional	microphones,	perhaps		 not	as	much	as	someone	with	less		 of	a	hearing	loss,	but	significant		 nonetheless.

Fine Tuning the Hearing Aid
Professionals	hold	different	views	 about	using	this	feature	for	people	 with	severe	hearing	loss.	Decisions	 that	have	to	be	made	concern	the		 degree	of	compression	to	utilize,	at	 what	intensity	level	it	should	begin,	 and	how	quickly	or	slowly	it	should
HLAA Hearing Loss Magazine July/August 2010

Table of Contents for the Digital Edition of HLAA Hearing Loss Magazine July/August 2010
